Before Einstein developed his theory of Gravity, Newtonian Gravity was the widely accepted theory. Newtonian gravity works out mathematically for most instances.
Newtonian gravity states that the strength of gravity depends on the distance between two objects. Einstein found fault with this particular idea, since according to his theory of relativity, the distance between any two objects changes based on an observer’s reference frame. Thus, Einstein set force to develop a gravitational theory cohesive with his theory of special relativity. After years of work on “General Relativity,” Einstein concluded that gravity is the result of a curvature in a four-dimensional fabric that makes up our universe, which he termed “spacetime." Space and time are not the distinct and absolute qualities we perceive them to be, according to Einstein. Rather, “Three-Dimensional Space” and “Time” actually exist as a single continuum of four-dimensional “spacetime.” Mass curves this fabric of spacetime, similar to the way a heavy ball would pull down the center of a trampoline. If you were to roll a little ball on the trampoline with the heavy ball in the center, the little ball would be drawn toward the bigger ball would rotate around it in a circle. Normally, the little ball would follow a straight line, but the larger ball in the middle distorts the surface of the trampoline, and thus the path of other objects on it. This is analogous to gravity for us, but the earth replaces the big ball and the objects on and surrounding the earth replace the smaller ball. The earth warps spacetime and this causes the inward pull of gravity that we experience on earth.