A mask is a disguise that covers just your face. Your Halloween costume might include a scary monster mask and a furry suit with a tail, or maybe you just wear that for fun.
You can wear a mask to conceal your identity, to dress up as a character for a party or holiday, to scare someone, or to make someone laugh. Robbers sometimes wear masks so they can't be identified, and an actor might wear a mask while playing the role of a rabbit in a play. Other masks are useful, including masks that protect patients from germs and masks that keep skiers' faces warm.