The idea is that there is a stream of music flowing through each level, and it's your job to use the various widgets you've been given to shape and paint the music into certain spots on the board. As you move through the levels, you'll get more and more adept at sending the flow where you need it to go. Fortunately, the puzzles get harder and harder as you play, creating a great curve of both learning and adjusting your mind enough to find the right solution.
The main game is free, and then you can purchase all of the levels in three different packs, at 99 cents each. But even for the whole three bucks, this is quite an experience; it's more than worth the price. Again, if you're doubtful, give the demo a try, and see if you're not convinced to try the whole thing in order to see where it goes.