He established that gravity is not instantaneous, because nothing can go faster than the speed of light. Einstein recognized the potential in studying gravity.
He's the one that came up with the General Theory of Relativity.
So what is this theory?
First of all, imagine the Universe is a giant sheet representing "space-time." Now imagine putting a big ball on this fabric and watching it ripple like pond water. Now pretend that the ball represents a giant mass, like a planet. The steepness of the dent caused by the planet is gravity! The mass essentially warps space and time. The bigger the mass, the bigger effect it has on the fabric of space-time.
The mass affects the size of the warp.
In a nutshell, his theory says that gravity is caused by masses causing a dent in this space time fabric.