Mustn't is a contraction of “must not,” meaning something is forbidden or strongly advised against. When your teacher says “You mustn't run in the hallways,” she means running isn't allowed. When a recipe warns “You mustn't open the oven door while the soufflé bakes,” it means doing so will ruin your dish.

The word carries more weight than simple “don't” or “shouldn't.” Mustn't suggests real consequences or rules that matter. A lifeguard doesn't say “you shouldn't swim during a lightning storm”: they say “you mustn't swim during a lightning storm” because it's genuinely dangerous.

You'll see mustn't more often in British English than American English. Americans tend to say “must not” or “can't” instead. When you read classic British children's books, you'll notice characters saying things like “We mustn't be late for tea” or “You mustn't tell anyone our secret.” The contraction can sound a bit formal and old-fashioned to American ears, but it's perfectly normal in Britain and other English-speaking countries.