Needn't is a contraction of “need not,” meaning there's no necessity or requirement to do something. When your teacher says you needn't bring supplies tomorrow because the school will provide them, she's telling you it's not necessary. When a coach says the team needn't worry about the upcoming game, he means there's no need for concern.

The word signals freedom from obligation. If you finish your homework early and your parent says you needn't do extra problems, they're releasing you from that task. It's subtly different from saying you shouldn't do something (which suggests it's a bad idea) or you can't do something (which means it's not allowed). Needn't simply means the requirement doesn't exist.

You'll encounter needn't more often in British English or formal writing than in everyday American conversation, where people tend to say “don't need to” instead. A British friend might text “You needn't bring anything to the party,” while an American friend would more likely write “You don't need to bring anything.” Both mean exactly the same thing: showing up is enough.