Definition
Special relativity is a physics theory built on two principles: the laws of physics are the same for every observer moving at a constant speed, and the speed of light is always the same for everyone. From these, it shows that time runs slower and length shrinks for objects moving fast. Albert Einstein published it in 1905, and it changed how humanity thinks about time and space from the ground up.
Why doesn't the speed of light change?
Imagine you're on a train moving at 100 km/h, and you throw a baseball forward at 50 km/h. To a person standing still outside the train, the ball seems to fly at 150 km/h, the two speeds added together. In our everyday world, the speeds of moving things simply add up like this.
But light doesn't follow our common sense at all. Say you're on a spaceship flying close to the speed of light and you switch on a flashlight pointing forward. To the people on the ship and to people standing still outside, the light travels at the very same speed: about 300,000 km per second. The ship's speed never gets added to the flashlight's beam to make it faster.
Whether an observer is standing still or racing along near the speed of light, the speed of light is always the same for everyone in the universe. Einstein didn't treat this strange fact as a trick of the eyes. He accepted it as an absolute law of the universe, and from there the surprising secrets of time and space began to unravel.
The magic of stretching time and shrinking length
Speed is the distance traveled divided by the time it took. So if the speed of light has to come out as the same number no matter how you measure it, what has to change? In the end, distance and time themselves must stretch or shrink depending on how the observer is moving, so that everything adds up.
Suppose you watch a spaceship zoom past at close to the speed of light. The clock inside the ship looks like it's ticking noticeably slower than our clock out here. Physicists call this mysterious effect "time dilation."
That's not all. The ship's appearance changes in a surprising way too. Along the direction it's flying, the ship looks squeezed, front to back. This is called "length contraction." The closer the speed gets to the speed of light, the more time slows almost to a standstill, and the more flat the length becomes.
A bit more precisely: E=mc² and the speed limit
To be a bit more precise, the closer an object gets to the speed of light, the harder it is to speed it up, no matter how much energy you pour in. Pushing an object with mass all the way to the speed of light would take infinite energy, more than all the energy in the whole universe. That's why nothing with mass can ever go faster than light.
Along the way, Einstein found that mass and energy are really one thing, like two sides of a coin. This is the most famous equation in physics, the mass-energy equivalence formula (E=mc²). It means that even a tiny speck of mass, multiplied by the enormous number that is the speed of light squared, can turn into an unimaginable amount of energy.
The theory is called "special" because it covers only a special case: observers moving at constant speed (inertial frames), with gravity left out. It can still calculate the motion of accelerating objects. Later, Einstein widened his thinking into general relativity, which includes gravity and acceleration and says that spacetime curves.
Common misconceptions
A person on a fast-moving spaceship can feel their own time slowing down and their body getting flattened.
To the person on the ship, everything is completely normal. Their clock ticks once a second as usual, and their body is the same length. Time dilation and length contraction are relative differences seen only by an observer watching the ship from outside.
Where you meet it
GPS satellites orbiting Earth have clocks that slow down because of their high speed. But they orbit high up where gravity is weaker, and that makes their clocks run faster by an even larger amount. The net result is that they gain tens of microseconds per day compared with the ground. So the satellite clocks are set before launch to run slower by that amount, which cancels the difference.
Muons made high in the atmosphere have very short lifetimes. But because they fly at close to the speed of light and their time stretches, they manage to reach detectors on the ground.
In one sentence
Because the speed of light is always constant, fast-moving objects have slower time and shorter length, and mass is itself a form of energy.