It is really about several things; but briefly, when you go at high speeds, several things change (depending on the point of view). The things that change include how fast time passes, measures of distance, and masses. Also, through their gravitation, large masses cause certain distortions in both space and time. Strange as this theory is, different aspects of the Theory of Relativity have been confirmed through countless experiments.
For a more thorough introduction, read the Wikipedia article on "Theory of Relativity".
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The most famous one are his Theory of General Relativity, and Theory of Special Relativity.
Created the theory of relativity and explained the photoelectric effect
In 1905, where it was published in Einstein's theory of relativity.
He invented atomic power He came up with the theory of Relativity. He came up with the theory of Gravity. He had all these equations like E=mc2
No he is known for General relativity, Special relativity, Photoelectric effect, Brownian motion, Mass-energy equivalence , Einstein field equations,Unified Field Theory, Bose-Einstein statistics