What is need not?

Need not is a modal auxiliary verb that indicates lack of obligation or necessity. It is commonly used to express a negative permission or advise someone that they are free to do something without fear of negative consequences.

Examples:

• You need not worry about the deadline; you still have plenty of time.
• He need not go to the meeting if he doesn't want to.
• You need not answer the phone if you don't want to talk to them.

Note that 'need not' is different from 'don't need to' or 'do not have to', which also indicate lack of obligation, but do not necessarily convey the same level of permission or freedom.