The Grand Unified Theory is a vision of a physics theory that can combine three of the four fundamental forces into one single equation. The four forces are the Strong Nuclear Force, the Weak Nuclear Force, the Electro-Magnetic Force, and the Gravitational Force. The EM and Weak forces were initially thought to be two separate forces until scientists discovered one theory (the Electro Weak theory) to explain both of them and then went on to observe this unified force in action (much like Maxwell unified the electric and magnetic forces into the Electro-Magnetic Force).
If a Grand Unification of all the interactions is possible, then all the interactions we observe are all different aspects of the same, unified interaction. However, how can this be the case if strong and weak and electromagnetic interactions are so different in strength and effect? Strangely enough, current data and theory suggests that these varied forces merge into one force when the particles being affected are at a high enough energy.
The grand unification energy is the energy level above which, it is believed, the electromagnetic force, weak force, and strong force become equal in strength and unify to one force governed by asimple Lie group. Specific Grand unified theories can predict the grand unification energy but, usually, with large uncertainties due to model dependent details such as the choice of the gauge group, the Higgs sector, the matter content or further free parameters. Furthermore, at the moment it seems fair to state that there is no agreed minimal GUT.
The unification of the electroweak forces and the strong force with the gravitational force in a so-called "Theory of Everything" requires an even higher energy level which is generally assumed to be close to the Planck Scale. In theory, at such short distances, gravity becomes comparable in strength to the other three forces of nature known to date. This statement is modified if there exist additional dimensions of space at intermediate scales. In this case, the strength of gravitational interactions increases faster at smaller distances, and the energy scale at which all known forces of nature unify, can be considerably lower. This effect is exploited in models of large extra dimensions.
The exact value of the grand unification energy (if grand unification is indeed realized in nature) depends on the precise physics present at shorter distance scales not yet explored by experiments. If one assumes the Desert and supersymmetry, it is at around 1016 GeV.
The most powerful collider to date, the LHC, is designed to reach a center of mass energy of 1.4x104 GeV in proton-proton collisions. The scale 1016 GeV is only a few orders of magnitude below the Planck scale, and thus not within reach of man-made earth bound colliders at the current moment.