Einstein's theory of Relativity is built off of the fact that the speed of light is constant in all inertial frames. That means that if you are standing still, the speed of light relative to you is 3 * 10^8 m/s. And if you are moving at 2*10^8 m/s, the speed of light relative to you is still 3 * 10^8 m/s. This has been proven true by experimentation, so then we have to come up with a theory to explain it. It turns out that this with this new theory, not much changes until you move at 0.1c (c is the speed of light). But as long as the speed of light is always constant, then all these other weird things happen. Like when you observe something that is moving very fast relative to you, it gets shorter (length contraction). And when you observe something that is moving very fast relative to you, its clock is slower (time dilation). And then there's the doppler shift, which is where the frequency of perceived light is altered by the reference frame of the observer. Basically,