While on first blush this might seem very useful for someone who is overseeing a number of editors, sadly it only shows the log for yourself.  So changing Users to "All users" does not do much.

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1. Users - What user's actions are you viewing.  All users or self.  This is deceptive as at the moment only admins can view other users than themselves.  
2. Depth - How many levels do you want to see the log for?  Current page through 3 levels
3. Time - How long a period do you want to see the log for?  This week, Last week, Last 7 days, This month, Last Month, Last 31 days, No Limit. 
4. The table of information displayed
   • Time - 24 hour time
   • User - the user @Live  means the user is in the Live workspace, @Draft means the user is in the Draft Workspace
   • Action - Update, Insert, Check (seems to be used with moving), Delete
   • Table - what table the action was on (usually pages or tt_content)
   • Details - While cryptic you can see what pages or content elements were affected (see below) as well as access the history/undo for each item
5. Show History - Access the history and undo

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The highlighted sections show that the content was changed in one entry and pages in another.  The Action column shows what action occured.