His Uncertainty Principle, discovered in 1927, states that the determination of both the position and momentum of a particle necessarily contains errors, the product of these being not less than a known constant. These errors are negligible at everyday dimensions but paramount when studying small and fast particles.
Heisenberg remained in Germany during World War II, working under the Nazi regime. The extent of his cooperation has been a subject of controversy.
Nuclear fission was discovered in Germany in 1938, and given Adolf Hitler's obsession with warmaking, Germany might possibly have produced an atomic bomb had the Nazis made that a high priority. During the war, Heisenberg was the most prestigious German scientist, and his advocacy for an atomic bomb might have convinced Hitler to proceed. Heisenberg did other sorts of work during the war and the Germans produced no atomic bomb. Some have called Heisenberg an evil man for collaborating with an evil regime; others have praised him for steering the Nazis away from atomic weapons.
Heisenberg himself said little on the matter. His pride would not allow him to admit he had made any drastic mistakes during the war years, but he seemed reluctant to claim wisdom or bravery either.