A non-disclosure agreement is a signed promise that a prospective buyer will keep confidential the financial and operational information a seller shares during due diligence, and will not use it for any purpose beyond evaluating the deal. Sellers of a business typically require one before releasing anything beyond a basic summary, since employees and customers finding out too early can hurt the business.
Why it matters
An NDA is what lets you show a serious buyer your real numbers without broadcasting that the business is for sale.
General information, not legal or tax advice.