Black hole is created by collapse of a dying star with sufficiently large mass. Because of the huge gravity, the star collapse within itself and creates Black Hole. Though Black hole is infinitely small in volume, it has unimaginable huge mass. Because of the intense gravity of Black Hole, even light cannot escape from it. There are millions of Black Holes in our Milky Way galaxy.
If our Earth somehow goes too close to a Black Hole, Black Hole's intense gravity will consume our Earth. Certainly we don't want to be in Earth in that moment. As our universe is unimaginably huge, the chance of our Earth colliding with a Black Hole is very low.