Theory of Relativity

Theory of Relativity consisted of 2 theories:

1. Special theory of Relativity
2. General theory of Relativity

Special Theory of Relativity:
Consists of two principles:
1. The law of physics are relatively the same in uniform motion
2. The speed of light are the same in vacuum for all observers or the source of light

For (1), for the law of physics that is valid for a particular system A, also valid the a system A', which is uniform translatory motion relative to A.

For (2), why we care about the speed of light be the same for everyone?

General Theory of Relativity

In a simple term, general theory of relativity tries to solve the problem of gravitational force.
The Euclidean geometry need to be abandoned in favor of "space curvature". The idea that a "straight line" is a straight line does not apply in general - it could bend somehow.

For example, light rays are curved by the action of gravitational fields.

If that is true, you can imagine a uniformly rotating body that produced a centrifugal force is indeed, the gravitational force.