To be honest, I don't even know what's so fun about it, but it's one of the most addicting games I've played. Like you said, you need no musical knowledge nor do you need to know how to play guitar.
The controller looks like this:
depending on what console you play it on.
The game consists of "notes" scrolling across the screen. You have to play the notes by pressing the corresponding colored button and string button as they scroll across. The notes correspond to the guitar part of whatever song you're playing. Here's
an example in video since it's sort of hard to explain (that video is the hardest song in the game on the highest difficulty, so don't worry...it's not that hard).
So as you can see the "notes" really have nothing to do with musical notes, therefore no musical knowledge required. Like I said, it's hard to explain. The best way to find out is just play it, because almost everyone who has comes back saying it was a blast and they want to go pick it up for themselves.