The reason we use the word "relativity" to talk about Einstein's ideas is because he tells us that everything is always moving. (Even if you stand very still you are moving with the rotation of the Earth and the Earth's orbit of the Sun and the Sun's movement.) All observers see things from their own frame of reference. So if a car is moving away from you, it's just as accurate to say that, to the people in the car, you seem to be moving away. There is no absolute frame of reference. It's all relative.
Einstein did talk about one constant: the speed of light in a vacuum. It's always the same, no matter how you observe it (or how you are moving when you observe it.) And matter can never move at the speed of light. This is tied in with some other important ideas:
Space and time are part of the same thing, called spacetime. You can't have one without the other. And although space and time seem like absolutes to us (It seems as though a foot is always the same length and a second is always the same length of time) they can change.
Mass - all the stuff in the universe - and energy - all the forces in the universe - are related. Energy can become mass and mass can become energy.