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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 (which would seem to make 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 carried out tests that effectively checked 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, such as some movement along the direction the light is traveling. Einstein worked out the logic of how this could be both true and consistent, which was eventually termed relativity, and later termed special relativity after Einstein additionally worked out general relativity. Both virtually match Newton's laws for speeds far less than that of light, but significantly diverge for speeds closer.
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 adopting a notion that goes against things we've "known" our whole life about how physical reality works. 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 an Earth-like apparent gravity) required Einstein years to devise and he used a mathematician's assistance to learn the required math.
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 within.