A blanket is a large piece of material that keeps you warm when you're in bed or taking a nap on the couch. Many small children have a favorite blanket that provides not just warmth, but comfort.
There is the knitted blanket on your bed, and then there's the blanket of snow that covers your yard in January. Any heavy layer can be described this way: "After a windy day, there was suddenly a blanket of leaves outside." You can use it as a verb, too: "The fog descended and began to blanket the whole town." The Old French root is blanchet, "light wool or flannel cloth," from blanc, "white" or "white cloth."