Einstein went back to the drawing board and puzzled over incorporating accelerated motion into his theory of Special Relativity so that he could account for things like Gravity. Eventually what he realized was that space and time were being bent by gravity. The stronger the gravitational force, the stronger that space and time were bent. He also realized that time in different levels of gravity runs at different rates. That is to say, the higher gravity is, the slower time flows for particles within that gravity field. This is Einstein's General Theory of Relativity.
Imagine you have a molecule undergoing radioactive decay. As you may recall from high school, radioactive decay happens with a logarithmic curve. At the beginning, lots of particles are emitted, but over time, the rate at which they are released slows down. Scientists can't precisely say when all of the particles will be emitted, but they can estimate fairly accurately when half of them will -This is what is known as the half-life. What they have found though is that if you are in higher gravity, the half-life is longer, because time runs slower.
This effect is so pronounced that it even affects GPS satellites orbiting the earth. If they didn't take relativity into account, they would be off by a significant amount. We are literally living in a slower timeframe than stuff floating around in space (we experience more gravity here than they do up there). Think about that at your next birthday. If you were in space, you'd already be older than you are now. Not by much. Even after 20 years in space, you wouldn't even be a year older.
Einstein realized something while creating General Relativity. Imagine you are inside an elevator (so you can't see outside of it), and you drop an object and it falls to the ground. You must be on earth, right? Not necessarily. You might be in space on a rocket that is accelerating. It acts similar to gravity, even though it isn't. Now imagine you go to drop the object but it just floats away instead. You must be in space, right? Not necessarily. You might be in an elevator that is dropping down an elevator shaft. Falling can be a lot like being weightless in space.
One common misconception about space is the concept of weightlessness. We see astronauts floating outside their ship and think that they must be experiencing no gravity. Not true, they are. Astronauts up on the international space station experience gravity just like you and me, but the difference is that they are also moving really fast horizontally. As in more than 17,500 mph.
Imagine you have a cannon and you fire a cannonball. It will make a curve out and then down as gravity pulls the ball down. Now imagine you crank up the power of the cannon. As you increase the power, the ball goes further and further. Eventually, when you crank the speed of the ball up above 17,500 mph, it goes so far that when it begins to curve downwards, the earth is also curving downwards. The ball keeps falling, but the earth keeps curving down, so the distance between the ball and the earth will actually decrease only by a small amount, none, or it will actually increase, even though the ball is still falling - the reason being that the earth is curving downward faster than the ball is falling.
So I've talked about Special and General Relativity, and you are wondering what this has to do with black holes? That's coming up next.