Spammers, trolls, and ban evaders often use temporary email addresses. PieFed now checks a list of known temporary email providers and displays a warning icon next to registrations that use such services.
If registration mode is set to "Open" (no approval needed) then the site admin(s) receive a notification instead.
A throwaway email address isn't always a bad thing but it's one factor that admins might want to take into account.
It's just a warning, you can still register using it I'm pretty sure
Yeah. Just off to the right of the screenshot is the approve button which didn't seem relevant at the time.