When a recipient clicks "report spam" instead of unsubscribing, a mailbox provider that offers a feedback loop forwards that complaint back to the registered sender, identified by IP or domain, usually as an ARF-formatted report. Without an FBL, a sender only sees complaints indirectly through reputation degradation.
FBLs require registering the sending IP or domain with each participating mailbox provider individually; coverage and format vary, and not every provider offers one. Complaint data from an FBL should immediately suppress the reporting address, the same as an unsubscribe.
A complaint rate is one of the heaviest-weighted reputation signals mailbox providers use; a rate above roughly 0.1% of a send is enough to trigger filtering even with otherwise clean authentication.