Black holes are the celestial objects with such immense gravity that nothing can escape from them -- not even light!
Einstein's theory of General Relativity tells us that space and time are intertwined. This space-time can be curved and warped by anything that has mass, like the sun or earth. Gravity is not a force that attracts objects like magnetism, but is the result of this curve in space-time.
We can treat space-time as a stretched piece of fabric. If you were to place a bowling ball in the middle, the fabric would bend. Any object placed on the borders of the fabric would be drawn in. This is analogous to the way gravity acts.
Black holes are not holes. They are regions in space-time with an extraordinary amount of mass concentrated in a very small space -- like a dense bowling ball. There are different kinds of black holes. Stellar black holes have a mass equal to that of the sun, and arise from large dying stars. Supermassive black holes, present at the center of many galaxies, have a mass million times that of the sun and their origin is still unknown.
Since black holes draw in light, they're invisible. We know of their existence by indirect observation like observing the path of a star near a black hole. Learning more about them can open a Pandora box unraveling the mystery of our Universe.