The audit log is a record of who did what, and when, across your account. Logins, incident edits, closures, and permission changes are all captured automatically, without any manual logging required.
Only users with the appropriate administrative permission can view the audit trail — it's not visible to every user by default, since it can reveal sensitive detail about who accessed what.
It exists for two practical reasons: supporting an internal incident review when the sequence of edits matters, and answering a compliance or data-subject request about who accessed a particular record and when — both of which are far easier with a real audit trail than with memory alone.