How does the Readwise sync with Goodreads work?

You can connect your Goodreads account to Readwise here

Once you've connected your account to Readwise, we will pick up all books on your Read shelf. For every book on your Read shelf, we will check if we have any Supplemental Highlights for that book. If we do, we'll add it to your Supplemental Books library, and you'll start seeing popular highlights from those books in your daily review.

This can be incredibly useful when just getting started with Readwise and you don't have many highlights of your own, as you'll still be able to tap into the wisdom of things you read in the past.