What is the science behind the gravity of Earth?
There is a very easy analogy for how gravity works. Everything in the universe takes place within spacetime. Imagine spacetime as a trampoline. Where mass exists, spacetime becomes curved, just like a trampoline would become curved if one were to place a bowling ball on it. We as people fall down the curves made in spacetime, but the earth keeps us from falling any further, thus we are stuck to its surface. If one were to put a ping-pong ball on the trampoline, it would fall down the curve made by the bowling ball. It would stop once it hit the bowling ball, and be stuck to its surface. There are a few subtleties to this model that keep it from working perfectly, but this is basically how gravity works.