Superstring theory is a possible unified theory of all fundamental forces, but superstring theory requires a 10 dimensional spacetime, or else bad quantum states called ghosts with unphysical negative probabilities become part of the spectrum.
Now this creates a problem in d=10 string theory: how to get the d=4 world as we know it out of the theory.
So far there are two main proposals:
1. Roll up the extra dimensions into some very tiny but nonetheless interesting space of their own. This is called Kaluza Klein compactification.
2. Make the extra dimensions really big, but constrain all the matter and gravity to propagate in a three dimensional subspace called the three brane. (For an analogy, your computer screen could be said to be a two brane of three dimensional space.) These types of theories are called braneworlds.