I would love to teach an entire course on special relativity and general relativity. But alas, we do not have enough time to cover that much material. So, here is a quick primer in relativity.
In 1905, Albert Einstein, who was working as a clerk at the Swiss Patent Office at the time, published the Special Theory of Relativity. Without any experimental evidence (yet), Einstein proposed the theory of relativity which is based on two propositions: (1) the laws of physics are the same in all inertial reference frames; (2) the speed of light is the same in all intertial reference frames.
There are many important, experimentally-verified consequences of special relativity, but I will only focus on one result: the four-dimensions of spacetime.