What is grand unification theorem?

The Grand Unification Theory used to be called the Unified Field Theory. It is a search for a theoretical/mathematical framework that can account for and unite all the basic forces, and therefore be considered a Theory of Everything. Currently there are 2 important branches of physics which seem to contradict each other. General Relativity which explains the behaviour of big systems and Quantum Mechanics which explains the behaviour of small systems (atomic/sub-atomic.) If you apply the wrong theory for the size of system then predictions from the theory don't match experimental results. (i.e. The theory is wrong!) Many physisists think there is a theory that can adequately explain both of these systems but they haven't found it yet. Einstein spent his last years attempting to find it. He did not. M theory, a development of string theory is currently considered by many to be the best prospect for a Grand Unification Theory.