From Wikidweb

A new feature in MediaWiki 1.4 is the recent changes patrol. This is a feature open to any logged in user. It will highlight edits on recent changes which have not been marked as "patrolled" by another user. The aim is to decrease redundancy with many people checking the same edit.

Unchecked edits appear with an exclamation mark (!) next to them. When you click a diff link from recent changes, you will see a [Mark as patrolled] link. If the edit is not vandalism, click the link to remove the exclamation mark from recent changes. If the edit is vandalism, you should revert it.