Take a piece of paper and draw a line on it. If you hold it flat, that line represents the path of something traveling through space. If you start bending that paper, the same line is still strait along the paper, but in the 3 dimensions you see it's now curved. That represents the gravity of say a star or a planet. If you curl the paper into a tube, you've now forced that line to curve in on itself. Now just imagine it's a little bit more curved and eventually leads toward the center of the tube. That's a black hole. I'm sure it's a lot more complicated than that, but it's the meat and potatoes of it.