A clam is a marine animal with two shells that eats by straining food from water, sand, or mud. You can sometimes find clams by digging in the sand close to the seashore.
The largest clams are used for food by humans, including the scallop, a particularly big form of clam. When you order soup in New England, it's not unlikely that it will be clam chowder, a creamy, thick stew with potatoes and pieces of clam. When clam is a verb, it means "to dig clams on the beach," and if your friend offers to pay you 1000 clams for your old car, she means "dollars."