I think, there are several variations of this. One variation was having a list of a hundred words and for each word a corresponding sketch was to be drawn. Another variation, I think, is offering 100 small sketch commissions ($5 or so each) and getting them all done at a decent speed, possibly within a month. I never really bothered to look closely into any of those so I'm not familiar with the commercial variations.

From the artists I've seen who have done the challenge, its A) A way to make a quick buck and B) A test of skill. More often than not its a mix of the two. Even so, its not that bad of a deal. I've nabbed a couple of these "100 Sketch Challenge" (also called the Iron Artist Challenge as well) because the artists doing it were offering sketches at a lower price or with more options than what they normally would.

It's exactly what it sounds like: you take 100 commissions of a fixed type and price level, and that's that.