It might not make sense to follow this pattern strictly, but it seems like the best place to start explaining how a debate round works is to talk about what happens in the first speech. So let's start at the beginning. The affirmative speaks first, and in their "First Affirmative Constructive" or 1AC they are tasked with building a case "affirming" the resolution.
There are many different approaches that the affirmative can take to affirm the resolution (especially considering the long history of policy debate), but the vast majority of affirmatives will present a plan. Most high school resolutions are very broad, for example this year's is "The United States federal government should substantially increase social services for persons living in poverty" There's a lot of different different social services the government offers, many different ways for the government to "substantially increase" them, and many different kinds of "persons living in poverty." The aff generally finds it to their advantage to advocate a more specific policy option, and presenting a plan is how to do that.
Let's say I'm the affirmative and I choose the following plan to advocate: "The United States Federal Government should expand Medicaid coverage to all persons meeting the United States federal poverty definition." I haven't thought too hard about the wording, but I'd expect that similar plans will be popular next year. Reading the plan itself won't take me much time, so I'd use the rest of my 8-minute 1AC to build a Prima Facie case. Prima Facie is a latin term borrowed from the legal system which means "on first appearance." Basically a prima facie case is a complete justification for my plan. There are four parts that make a complete affirmative case in policy debate, we call them The Stock Issues: Topicality, Harms, Inherency, Solvency. THIS is a popular pnemonic.
Topicality is whether or not the aff's plan fits under the resolution. The Aff gets to prepare their 1AC ahead of time, meaning they effectively get "infinite prep time." If the Neg came prepared to debate social services for poverty and the Aff's plan was to fund a NASA mission to Mars, it's not very fair to the neg. For my Medicaid plan, however, the aff has a pretty good argument that their plan is topical. I'll describe some of the specifics of how to argue T in a different article, but one important thing to note is that the aff's plan is generally assumed to be topical until the Neg argues otherwise. Most affs don't spend much time arguing T in the 1AC.
The second stock issue is Harms, or the reason why we need plan. This is usually a reason why the Status Quo - more latin, for "the way things are right now" - is bad. There are a bunch of different "harms" that apply to persons living in poverty, but our case would probably focus on a lack of access to health care. You can live "under the poverty line" and still not qualify for Medicaid, especially if you don't have any kids. Many of these people have jobs and able to meet their basic necessities, but would be unable to pay for any type of medical emergency. Something as simple as breaking an arm in a car accident could cause them to spiral into bankruptcy or worse.
Inherency is a little bit trickier, mostly because it has a bit of an archaic name. Inherency is whether or not the harms are "inherent" to the status quo. The way I like to put it is, "If the plan didn't happen, would the harms continue?" As an affirmative, there are two possible attacks I'd need to defend against. First of all, I'd need to make sure that my plan, as written, won't happen on the status quo. If my plan had just passed (or will soon pass) through both houses of congress and was unlikely to be vetoed by the president, I'd have some serious inherency problems. I'd also want to show that no other action being taken in the status quo is likely to solve my harms. For example, if an insurance company were to start a low-cost insurance program targeted people below the poverty line who are uneligible for medicaid, I'd probably want to present some evidence that that program is not likely to succeed.
Solvency tends to be the Stock Issue that attracts the most argument. After all, it's easy to find problems, but generally much harder to solve them. Solvency is whether or not the affirmative's plan will improve the status quo or "solve the harm." My Medicaid plan would solve by offering the government's Medicaid insurance to everyone who makes less than the Federal Poverty Line. This expansion of coverage would reduce preventable disease and lower the bankruptcy rate. If this was a real round, then attaching some data and warrants to those claims would be a priority for my 1AC.
I've got plenty more to say about all of the stock issues, but that serves as an introduction. These are the components of an affirmative case, but they are also the first tools that the negative can use to win the round. If the Negative wins that, say, the Aff's plan doesn't solve (maybe medicaid coverage isn't enough to meet the needs of this demographic), then the neg wins the round.