A crater is a roundish dent left in the surface of something by the impact of something else. Look at the moon: it's covered in craters from thousands of impacts over millions of years.
Once in a while, you might hear the word crater used as a verb, as in, "His career really cratered when he started drinking. But you'll usually hear crater used as a noun to refer to a bowl-shaped depression left behind by an explosion or impact. Outside Flagstaff, Arizona, you'll find Meteor Crater, one of the most well-defined impact craters on the surface of the Earth. It's almost a mile across, even though the meteor that created it was less than 100 yards in diameter.