What is the Theory of Everything?

Theory of everything is a hypothetical theory in physics, which is supposed to explain all kind of phenomena by a single mathematical equation.

For example:
There are 4 fundamental forces in the universe namely,
1. Gravitational force
2. Electromagnetic force
3. Weak force
4. Strong force

At present, gravitational forces are explained by Einstein's "General Theory of Relativity" and the rest of three forces can be explained by using a single theoretical framework known as "Quantum Field Theory".

But still there is no theoretical framework which can explain all four forces using a single mathematical equation.

Theoretical Physicists are trying to develop a theoretical framework which can explain all four fundamental forces all together.

Some of the potential candidates to become ToE (Theory of Everything) are, String Theory and M-theory. Which try to explain all the forces by a single equation, but there has been no experimental/observational proof for these yet.

Let's hope our beloved theoretical physicists will find out the ToE, in couple of years in future.