The admin log records the details of the changes made to the site by all users. This log can be found in the admin console (Site Admin | Admin Log) and includes records about the creating, deleting, and changing of:
Pages and global content blocks
Templates and stylesheets
Users, groups, and permissions
Site preferences
Modules
At the top of the log, you will find a navigation section providing links to the first, next, previous, and last pages of the log. There is also a link to download the complete log as a tab separated text file. At the bottom is the Clear Admin Log link. This will clear the entire log.
Note
Important! You won't be asked to confirm the Clear Admin Log, and this action cannot be undone once completed.
Once the admin log is cleared, there is no evidence of who has changed anything on the website and what the changes were. Therefore, there is even a separate permission called Clear Admin Log that is granted separately from the permission Modify Site Preferences...