An audit log is a record of important actions in a system. It may note what changed, when it happened, and which account made the change.
Imagine a notebook at a shared workshop that records who borrowed a tool and when it was returned. If something is missing later, the notes help people understand what happened.
An audit log can help with troubleshooting and accountability. It should avoid storing unnecessary private information, and it is only useful if the records are protected and kept accurately.