Einstein's Theory of Relativity is his interpretation on how matter and energy are related, as well as the properties of the fabric of spacetime with respect to gravity. The General Theory of Relativity mainly covers the behavior of the fabric of spacetime around matter, because all matter has mass and therefore gravitational pull. The Special Theory of Relativity addresses the speed of light.
According to the Special Theory of Relativity, the speed of light is constant everywhere at approximately 300,000 kilometers per second. Also, there is no absolute position or frame of reference anywhere in space, so the speed of light is the same for all frames of reference. When combined with the General Theory of Relativity, it explains why lightspeed travel is impossible: objects require more energy to accelerate, and the amount required increases in a logarithmic scale toward infinity.
The Theory of Relativity also posits that space and time are a single combined dimension rather than separate components, Faster movement in space leads to slower travel along time's arrow, at least from the point of view of an outside observer. If two perfectly synchronized clocks were separated with one orbiting Earth, the orbiting clock would appear to have lost time. The reverse is also true.