A Non-Disclosure Agreement (NDA) is also known as a confidentiality agreement which is a legal contract between you and another party which prevents them from sharing your information with others.
To be able to share new knowledge and original work it required a lot of trust. It is essential to consider confidentiality before seeking a company to develop your ideas.
You will typically get an NDA signed when speaking to new manufacturing suppliers and rapid prototype companies. Basically whoever you choose to disclose information to you should get a NDA written up and signed.