special relativity
Special relativity is Einstein's 1905 idea that the laws of physics work exactly the same for everyone moving at a steady speed in a straight line — and that light always travels at the same speed, no matter how fast you or the light source are moving.
That second part sounds harmless, but it forces something strange: to keep light's speed fixed for everyone, time and space themselves have to bend. A fast-moving clock ticks slower than yours (time dilation), and a fast-moving object is squashed shorter along its direction of travel (length contraction). Both effects are real, just far too tiny to notice at everyday speeds.
It's called 'special' because it covers only the simple case: steady, unaccelerated motion, with no gravity in the picture. The fuller theory that adds gravity and acceleration came a decade later and is called general relativity.
This isn't just abstract theory: your phone's GPS would drift miles off course each day if engineers didn't correct for relativity on the orbiting satellites. Their speed makes their clocks tick a little slow — that's the special-relativity piece — while sitting higher up in weaker gravity makes them tick faster, an even bigger effect from general relativity. Together it adds up to about 38 millionths of a second a day, and ignoring it would throw your location off by miles within hours.