einstien defined space-time, and gravity is explained in the warping of space-time. think of it like standing on a huge trampoline. there is you and say a person who has twice your mass (you can say weight if you wish because you are both being effecting by the acceleration do to gravity, so the mass is proportional to the weight). now if you were to stand on the edge of the trampoline and the other person was to stand in the middle. as you get closer to the other person you can feel the effects of him on the trampoline, and in a sense with each step that you get closer, your ballance is thrown off more untill you get pulled into him. like this masses in space-time end up warping it, and the warp is so much that it pulls at it, hence gravity. now, there is even more deep explainations involving angular momentum, torque, and dark matter when deeling with planetary orbits, but this is the jist of it all