It's the worst fucking thing for an artist to ever encounter. It's when you run out of ideas to draw things off, and temporarily loose all motivation to do anything art related.
You'll often see artists on deviantART not post anything for a while, and then come back complaining of an art block.
Guy1: Why haven't you uploaded anything on DA lately?
Guy2: I'm having a serious art block.
Bitch1: Where's my commission I ordered? Why aren't you done yet (blahblahblah)
Artist: I'm having an art block. I'll be done when I can -__-