Special relativity (SR) is one of two relativity theories by Albert Einstein, and is an explanation of how the speed of light comes to be measured at the same rate even if your measuring equipment is moving along with the light (which might be thought to measure it as slower) or moving in the opposite direction (making it faster). For centuries, there has been a notion that anything you do in a closed room that is smoothly moving at a constant velocity is exactly the same as within a room that is motionless, a notion that seemed largely true and possibly completely true. After scientists succeeded in measuring the speed of light, they eventually tested whether its measured speed adheres to this notion. Experiment showed light's speed measurement is the same no matter how your measurement equipment is moving, and apparently being within a closed room is not a factor. Albert Einstein worked out the logic of how this could be true and consistent, which was eventually termed relativity, and later termed special relativity after Einstein additionally worked out general relativity.
Special relativity equations involve only a bit of high school math, and Einstein may well have worked out their principles in a single evening, the real challenge having been taking on notions that go against things we've "known" about how physical reality works, for our whole life. In contrast, general relativity (which also explains out how the manner in which things work while stationary on the surface of the Earth is exactly identical to things done away from Earth but accelerating just enough to feel like Earth-like gravity) required Einstein years to devise and he used a mathematician's assistance to learn new math to do it.
Both special relativity and general relativity are deterministic, and they calculate a deterministic future-prediction given a current state, just as do Newton's laws. Though this is the case, there have been people who associated Einstein's relativity with the idea that objective reality is somehow different for different people, a notion termed relativism. Relativity theories are unrelated to relativism and purport a single reality that everyone lives in.