Whether you're a company executive or a product engineer, your product manager may at some point deliver (hopefully not in a BUFR manner) you a market requirements document (MRD). The MRD will typically include functional and nonfunctional requirements that the product must satisfy.
The nonfunctional requirements comprise the constraints on your product that ensure it will solve or avoid prospect problems. The nonfunctional requirements will typically express the constraints as conditions or measurements that must hold as the user uses your product.
In my next entry, I will give an example of a nonfunctional requirement.