A list of the most recently changed cards showing who made the change and when. Many healthy community websites, most notably wiki, make heavy use of this functionality to insure that updated content is reviewed and approved.
Most (but not all) Decko sites choose to display a recent link prominently in their navigation. Even if not, you can usually reach a recent changes page by going to /recent or /:recent.
If a card was created, the change listing will note that it was "added". If the content of a card was changed, it will note how long ago it was "last edited" (and clicking on those words will show you the change). If something else was changed about a card (eg. its type) it will note that it was "updated".