Comp titles
Comp titles are the recently published books a writer names to show where their own book sits in the market — who already buys this kind of story, and which shelf it belongs on.
The convention is two or three of them, published in the last three to five years, in the same category as your book, and successful without being enormous.
That last constraint is the one writers get wrong. Comping The Hunger Games or Harry Potter reads as inexperience, partly because those numbers are not a plan anyone can repeat, and partly because such famous books tell an agent nothing specific. A comp that is doing its job says: the people who bought this, in this quantity, would buy mine.
It helps to remember that a comp is an argument about audience, not a claim about quality and not a list of your influences. Naming Faulkner says something about your ambitions and nothing about the sales conference. The X meets Y formula works when both halves are legible to someone who has not read either.
Comps are also a revision tool. If you genuinely cannot find any, the book may be sitting between two categories — which is much better to discover now than after fifty rejections.