Needn't is the short form of need not. You use it when something is not necessary: nobody is forcing you, and it is fine if you skip it. 'You needn't bring a gift' means a gift is not required; it does not mean a gift is forbidden. Teachers in Britain still write and say this a lot. In the United States people more often say 'you don't need to' or 'you don't have to', and needn't can sound a little British or a little old-fashioned — though actually many textbooks in Taiwan still teach it as the neat pair with must: must = you have to; needn't = you don't have to. There is a past shape that trips people: 'You needn't have cooked' means you cooked, but it turned out there was no need — the cooking already happened. That is different from 'You didn't need to cook', which often means you stayed out of the kitchen. Needn't works like can and must, so after it you use the base verb: you needn't wait, not you needn't to wait.