His special theory of relativity is based on two postulates:
1. The principal of relativity: All laws of physics are same in all inertial frames
2. The constancy of speed of light: the speed of light in vacuum has the same value, c = 2.99792458 x 108 ms-1, in all inertial reference frames, regardless of the velocity of the observer or the velocity of the source emitting the light.
Postulate 1 is a generalization of the principle of Galilean relativity, which refers only to laws of mechanics. Einstein’s principle of relativity means that any kind of experiment – mechanical, thermal, optical, or electrical – performed in a laboratory at rest must give the same result when performed in a laboratory moving at a constant speed past the first one. Hence no preferred inertial reference frame exists.
Video about the two postulates: