What is relativity, in plain terms?
Two theories from Einstein, built on the observation that the speed of light is the same for everyone regardless of how they are moving — a fact that sounds harmless and forces enormous conclusions.
Special relativity (1905) applies to constant motion, and rests on two postulates: the laws of physics are identical for all observers moving at constant velocity, and light travels at the same speed for all of them.
Why this is so disruptive. If everyone measures the same light speed, and speed is distance over time, then distance and time must differ between observers to keep the ratio fixed. They are not absolute.
What follows:
Time dilation. A moving clock runs slow relative to you. This is measured routinely — particles that should decay in microseconds survive long enough to reach the ground because time passes more slowly for them at high speed.
Length contraction along the direction of motion.
Relativity of simultaneity. Two events simultaneous for one observer are not for another — arguably the deepest consequence, because it dismantles the idea of a universal "now".
Mass-energy equivalence, the familiar equation, meaning mass is a form of energy.
A speed limit, since accelerating anything with mass to light speed would require infinite energy.
General relativity (1915) extends this to gravity, and reconceives it entirely: gravity is not a force but the curvature of spacetime caused by mass and energy. Objects follow the straightest available path through curved spacetime, which we experience as attraction.
Its confirmed predictions: starlight bending around the Sun, observed in 1919; gravitational time dilation, where clocks run slower deeper in a gravitational field; the precession of Mercury's orbit; gravitational lensing; black holes; and gravitational waves, detected in 2015, a century after prediction.
Why you rely on it daily. Satellite navigation requires correcting for both effects — satellite clocks run fast from weaker gravity and slow from orbital speed. Uncorrected, positions would drift by kilometres per day.
It remains incompatible with quantum mechanics, which is the outstanding problem in physics.