on Jane Friedman site:

You’ve been slaving over your query, which might have taken longer than the manuscript. Your lists of agents in the databases and spreadsheets are all perfect. Even your synopsis looks right. But you have been avoiding a critical element of your project: comps.

Comps—short for comparison titles—are recent books that have something crucial in common with your book. Finding comps can be the most challenging part of pitching for many writers, and their ambiguity raises anxiety levels. How old is a good comp? Does it need to be the same genre as my book? What if there isn’t anything I can use that isn’t too old or obscure?

Squash your anxiety if you can and take your comps seriously. There are compelling reasons why comps count a lot. Comps are your strongest marketing tool, and the first chance you have to position your book in the market and in the world. An agent or editor should immediately understand something critical about your book based on your comps. They should help you articulate where you position yourself within a very competitive market. They are publishing shorthand, and you should speak fluent publishing when you enter the query trenches.