A release (or settlement release) is the contract that closes a claim: in exchange for the settlement payment, you give up all claims arising from the crash — known and, in most standard forms, unknown. Once signed, discovering a worse injury next month generally changes nothing.
This is why "quick settlement" offers deserve suspicion. An insurer offering money days after a crash is usually buying the release before the full extent of injuries is known.
Read before signing: who exactly is released (just the driver, or their employer too?), whether medical liens and bills are accounted for, and whether your treatment has genuinely stabilized. Signing is the one step in a claim that cannot be walked back.