What does a draw by Insufficient Material mean?

If you have just a king, and your opponent only has a king and a knight, he cannot mate you, no matter what. Not even if you helped him. So, you can claim a draw, and the computer will automatically accept, rather than watching him move that knight aimlessly around the board for an hour.

I think this site would draw the game immediately when the K v K+N position arose. FIDE calls this a "dead position". Neither player has sufficient material to mate the other.

There is a second rule concerning Insufficient Material and this applies when one player runs out of time. In order for the opponent to win the game "on time" he must have sufficient mating material on the board. FIDE and USCF rules differ slightly in this regard, and this site uses a simple piece count to determine if material is sufficient. So K v K; K+N v K; K+B v K are all insufficient.