When you walk into someone else's house, it's polite to wipe your feet on the mat — the pad that covers part of the floor inside the front door.
A mat is a kind of floor covering, often smaller than a rug or carpet. You might buy a sea grass mat to put in your bedroom, or a door mat for your apartment entryway. You can also use the word mat as a verb, to mean tangle or snarl: "The dog managed to mat the fur on his belly by rolling around on the beach." The Old English matte came from the Latin word matta, "mat made of rushes."