Even one small paragraph will help, something like, "When you click on the button, it will spin up a bunch of workers (1 for every 10 images). These will optimize the filesize of your images in parallel, and then push the changes back up to a branch and open a pull request for you to review.

There's no limit on the number of images - it can optimize hundreds quite quickly - 50 is the number of parallel workers you can have"

(i stole this off another of your comments and added filesize after optimise)