Please can someone try to explain this to a confused physics layman?

Firstly, I’ve probably got everything wrong, because although it sounds so awesome, I’m awful at this sort of thing…

I was reading how for string theory to work, you need the 11 dimensions suggested by (i think it was) supergravity. And then that all the membranes that are the universes that make up the multiverse in m theory (or something) exist within the eleventh dimension. But then, I saw someone talking about how the 11th dimension would have to be a trillionth of a mm wide, so what does that mean and how does it work?

I know I know nothing, but if you can try to tell me… =]