Human beings, by and large, are terrible at staying on topic. When you sit around with your friends, someone brings up the Presidential race, so then you talk about politics for a while. Someone mentions that he caught Obama's speech on the TV at the campus coffee shop, which reminds someone else of how she just discovered how awesome their mocha lattes are, which causes you to chime in about how you can't afford lattes anymore and you make your own coffee at home. Suddenly you're debating the relative merits of French Press v. Coffee Maker, when you started by debating Obama v. McCain.
And there's nothing wrong with that. It's in our nature to make connections, and it's fun to let a conversation wander between friends. However, what if we had to pick a winner and loser out of this debate? How do you care my argument about a french press leaving in the essential oils stack up against your point about the cost to businesses of Obama's cap and trade policy? Or, if we had agreed ahead of time to debate coffee makers, would it be fair if my opponent's 1AC talked about how awesome her cast iron teapot was?
Which is why there is a general rule in policy debate that "the affirmative's plan must adhere to the topic." But how do we determine whether or not a plan is on-topic, or in debatespeak whether it is "Topical"? For a more concrete example: what is the definition of poverty? The federal government provides one (aka "the poverty line")... do we use that one? What if the aff's plan is to raise the federal poverty line (IE extend our current slate of social services to more people)? That plan extends social services to people not "living in poverty" (by the federal definition, anyway). Does that mean it's non-topical? What if they prove that there are people who can't afford basic necessities even if they live "above the poverty line?"
All of these questions are up for debate. Think about that one for a second, because it's a powerful notion. In a competetive debate, the rules themselves are decided during the round. This is the first example most people encounter of a meta-debate, a debate about debate. There are many types of meta-debates (or theory debates, as they are more commonly called), but Topicality is perhaps the most important.
So how do you argue that the affirmative is non-topical? If you're the negative, there's a general form to follow to make a T argument. First of all, you construct an Interpretation of the resolution. Usually this is done by defining one or more words in the resolution. For the poverty T that I'm constructing, you might define "Poverty" by citing the government, who says that a single person making less than $10830 is "living in poverty."
The next step is to show that the affirmative is in Violation of your interpretation. For this plan, the violation argument is fairly straightforward. Raising the poverty line increases social services only to people who aren't living in poverty by the current definition.
Occasionally the violation argument is somewhat more complicated. For example, what if the Affirmative's plan does something topical, and something not-topical, for example: increase medicaid benefits globally, and also extend them to people above the poverty line. This might be an example of "Extra Topicality," or a plan that takes action in addition to a topical action.
Or perhaps the affirmative is only topical after considering the "effects" of plan. For example, the government could institute price controls on prescription medicine, which would mean that Medicaid recipients could buy more treatment at the same price, effectively a social service. This might be an example of "Effects Topicality," where a plan meets the topic only though a chain of events.
Both of these debates (Effects and Extra T) have some additional nuance to them, but I want to introduce the terms here.
The next part of a Topicality frontline (What you would read in a 1NC) is a set of Standards that defend your interpretation. Think of this as "reasons your interpretation is better than any possible aff interpretation." There are a bunch of different arguments you can use to show why your interpretation of the Topic is a good one, but they fall into a few general categories:
The Limits that an interpretation puts on the resolution are important. A looser interpretation of the resolution will allow more cases to be topical, meaning that the neg will have more cases that it needs to prepare against. A too-big topic would make life pretty hard for the negative; they would just have too much work to do to get ready for all the different aff cases. On the other hand, a too-small topic would get boring pretty fast, and the aff would have no chance to take advantage of their unlimited prep time.
But it's not just the raw size of the topic that's important. Ideally there should be a certain unifying theme to the affirmative's arguments, a "direction." We generally use the metaphor of Ground. Imagine we're about to have a snowball fight, your friends versus my friends. Before we start throwing snowballs we stomp a little line through the front yard. You point to one side and say "that's our side!" your team builds up some barricades on your side, my team builds up a little snow fort on my side. Then when we start our snowball fight, everyone knows what's going on. Duck behind this wall to take cover, throw snowballs over in that direction, watch out for snowballs coming from over there.
You might end up with some snow in the face, but everyone knows what's going on. Our game has some organization. If there wasn't a clear division between "my side" and "your side," it'd be chaos. We'd be throwing snowballs at our own teammates, nobody would know where to take cover, and someone would probably end up punching somebody in the face. Such is the case in debate. That clear division of ground is necessary to preserve the organization of the round. The negative (and the negative alone) ought to be able to advocate reducing social services for people living in poverty, for instance.
The Source of an interpretation also is somewhat important. There are some arguments for and against legal definitions, dictionary definitions, definitions from expert in the field, etc, but far more important is that a definition have intent to define. If I say that "During my years in college living in poverty, I consumed truckloads of Ramen," I don't actually intend to define poverty as including all college students. I'm using the word for effect, not in an earnest attempt to determine what is poverty and what is not.
So you've proven that the aff's plan doesn't meet the best interpretation of the resolution... what now? What should the judge do with that information? If you're the neg, you want Topicality to be a Voting Issue. In other words, you want to tell the judge "if we win Topicality, we win the round, regardless of other issues." There are a bunch of reasons that T ought be a voting issue, but there are two concepts that capture most of the reasons. Allowing the aff to run non-topical cases is a violation of fairness, and the only way to preserve the education value of the round is to have some focus on a topic. There's a lot to say about fairness and education as their own subjects, but those are the most important reasons that T should be a voter.
T is probably the most technical subject that new debaters have to learn right away, but don't be intimidated. There are some strategic implications that arise from whether or not you get a kick out of T debates, but you don't necessarily need to be a master of meta-debates right away. It's one of those things that you'll have to learn by doing.