Yeah, the basic idea is that instead of being a self contained universe, we are merely a set of different universes all existing on a membrane (hence M Theory). Now you could debate whether these universes are all in the same "place", as in they intersect our observable universe, or are they in some way separated.Humans have evolved to exist within 4 dimensions (the standard 3 dimensions plus time) so why would we perceive any others? So in a sense all the dimensions could exist "here" and they are interacting all around us and we wouldn't notice them.
The reason that this theory has been picked up is the problem with gravity. Despite being a dominant factor within the universe, you can argue that while gravity is capable of altering the speed of light (a big debate can be had about that one) it is also a very, very weak force. In physics there are 4 main forces; radioactive strong, radioactive weak, electro-magnetic and gravity. You can do a simple test to show how weak gravity is. We all know that the gravity of the Earth attracts us to it's centre, you jump you come back down again, however take a simple fridge magnet and attach it to the fridge door what happens? It sticks, right? Now look at the actual mass of the earth compared to the mass of the magnet. Despite having an infinitesimally smaller mass than the earth, the magnet (using the electro magnetic force) is able to defy gravity and keep the magnet attached to the door. With that in consideration, the question is why is gravity so weak in comparison to the rest of the forces. One theory is that gravity isn't weak at all, it's just that it's distributed throughout the separate dimensions, where the other forces are specific to this universe.