A novel synopsis is typically a three- or four-page summary outlining who the main characters in your novel are and what happens over the course of the story arc. (If you’re writing a nonfiction book, see the Nonfiction Book Proposal Critique option.)
Literary agents usually request these, along with a query letter, as a preliminary means of deciding whether your work may interest them.
A synopsis critique is $300 for up to 5,000 words. A synopsis critique will sometimes show problems in the manuscript itself, which means you’ll probably want to do a revision before continuing to query agents.