Albert Einstein never won a Nobel prize for the theory of relativity—in fact, it was only through long, political jockeying within the Nobel committee that he won the prize at all. Instead, when he was given the 1921 Nobel Prize in Physics (in 1922, after a long bout of internal Nobel hand-wringing), he received it primarily for his explanation of the photoelectric effect. Extraordinarily enough, he came up with both his relativity theory, and the photoelectric effect in the same year: 1905.
At the turn of the century, physicists already knew that, in some circumstances, exposing certain materials to light could create an electric current. An American named Charles Fritts had even created a working solar cell from selenium more than two decades before, in the early 1880s.
But observing that light can create electricity is not the same as understanding why light can create electricity. That was baffling.
It was understood, at that point, that light worked as a wave. But if that was true, it didn't make any sense that light could create an electric current: A wave of light just wouldn't have enough energy to cause materials like selenium to shoot off electrons as fast as they did when exposed to light.