Einstein's largest mathematical contribution was the general theory of relativity, as well as the necessary mathematical equations that explained it. His theory of relativity employed 16 equations, which are known as Einstein field equations.
Einstein's 16 field equations describe the gravitational effects produced by mass within general relativity. They describe the general theory of relativity, which explains that time is not objective, but rather dependent on an observer. One prominent equation that arose from the theory of relativity is what's known as mass-energy equivalence, which says that the mass of any system is the measure of its energy content.