if you’re familiar with the NASA ASAP program, FSAPs are basically ASAPs but for aircrews.
if you’re not familiar with the ASAP program, it’s when a pilot messes up and files a report saying “oops I messed up here’s what happened and what I should have done differently.”
as long as the pilot didn’t seriously violate any FARs or make the “mistake” intentionally, it’s a way to alleviate the chance of punitive action by showing you recognized the error and are going to prevent future mistakes.