What is a synopsis?
The synopsis conveys the narrative arc of your novel; it shows what happens and who changes, from beginning to end.
There is no single “right” way to write a synopsis. You’ll also find conflicting advice about the appropriate length, which makes it rather confusing territory for new writers especially. However, I recommend keeping it short, or at least starting short. Write a 1-page synopsis and use that as your default, unless the submission guidelines ask for something longer. Most agents/editors will not be interested in a synopsis longer than a few pages.
Read more via Back to Basics: Writing a Novel Synopsis | Jane Friedman.