What was Albert Einstein's most famous theory and how did it revolutionize our understanding of the universe?

Albert Einstein's most famous theory is the theory of relativity, which consists of two parts: special relativity and general relativity. Special relativity revolutionized our understanding of space and time by showing that they are not absolute but are interconnected and can be affected by motion. General relativity revolutionized our understanding of gravity by explaining it as the curvature of spacetime caused by mass and energy. These theories have had a profound impact on physics and cosmology, shaping our understanding of the universe at both the smallest and largest scales.