In a nutshell, a business owner will ask you to sign an NDA before a deal can commence. This allows the business owner to keep certain information confidential, including client lists, employee information, intellectual property, trade secrets, pricing structures and so on, in case a deal cannot be met. Typically, NDAs are often mutual - meaning, just as you're agreeing to keeping the seller's business information confidential, the seller must also offer that same level of confidentiality to keep your information private.
Another thing to look out for is non-solicit provisions. If a seller has this clause in their NDA, you are prohibited from soliciting any of their clients or employees for yourself or your company for around 1-2 years.