A functional requirement is one that the product must have to provide the capability needed by the ultimate user. Actually, functional requirements are the fundamental basis for the product in the first place. If a product does not provide a function, do a job, or complete a task, then the need for it goes away. An example of a functional requirement statement is:
The product shall produce an amended resource availability roster at the end of every work shift.
This statement says that for the product to be serviceable to the user, it must provide, among other things, the capability or function of keeping track of and reporting on the available resources each time a work shift comes to an end.
Generally, a description provided by the customer will yield several, even hundreds, of functional requirements, depending on the complexity of the product. But for every functional requirement, there also can be one or more nonfunctional requirements.