The audit log tracks user and admin activities across Microsoft 365. For example, if you need to find out if a user viewed a specific document or purged an item from their mailbox, you could find that activity in the audit log. You can use the Microsoft 365 Compliance Center to search the Unified Audit Log to view user and admin activity in your organization.
As of March 2021 here are over 650 audit activities available in the audit log with more added frequently. These are the main categories of information:
User activity in SharePoint Online and OneDrive for Business
User activity in Exchange ...
