It's a musical about an inept, but lovable supervillain who is trying to take over the world and win the affection of a girl from his laundromat he has a crush on. It's really funny.
Here's the first part of it:
I guess you can't really go into it with much expectations. It's like Dr. Horrible's Sing Along Blog, if you go into it expecting the most amazing thing in the world, you'll probably be underwhelmed.
You should watch it sometime, it's amazing. But maybe not the first two episodes, they're not as great as the rest. I'd watch Sonic Rainboom or Winter Wrap Up first.