The noun coast describes the area where the land meets the sea — the seashore. When your mom says, "We're going to the coast," pack your bathing suit, because you'll be near the ocean.
Coast also has a verb form that describes an effortless movement, like when you're riding your bike down a hill and you don't have to pedal at all. It can also be used figuratively to describe something that's gained without much difficulty. You might coast to victory in the playoffs when your team wins 8-0 and is never threatened.