Einstein put forth the idea that Gravity instead of being a force like newton said,instead of that Energy and thus Mass (Which is a form of energy altough not the only one), creates localized wrinkles or curves in spacetime.
Basically Spacetime means that Time and the 3D World you are seeing are interconnected. Thats important because conventional physics treated Time as its own separate entity.
How big of a deal? Imagine a Blanket suspended in air that is as wide as a football field.
Now you take a single marble and put it in the middle.
Not much happens.
Now you take a bowling ball and put it in there. You will see that the blanket gets pulled down, thus changing the way things on the blanket behave.
Now if you would throw a marble into the thus created funnel and somehow manage to put enough energy into it while holding it at a stable course? Well the thing would "Circle around" the bowling ball.
And to a degree that why things rotate around other things in our solar system. But if you keep in mind that time is also affected by this, that brings up an interesting point:
If a high amount of gravitational force can distort the fabric of space, what effects would that have on Time?
Well there is a simple example that we can point at.
We can not, despite many people thinking otherwise, see black holes. What we can see at best is the so called "Even horizon". Its that glowing ring around the black hole you see in the movies.
That light? Yeah it has been there forever. The gravity is so strong that it bends the light rays, forcing it to go in circle around it. And now it gets freaky: Since the Black Hole is so massive, the light that runs around it never stops and apperantly never decays. So at least the mass is able to slow down time to allow the light to be captured virtually forever.
SCIENCE