in which we look at those requirements that cause the product to do something
Functional requirements specify what the product must do—the actions it must perform to satisfy the fundamental reasons for its existence. For example, this functional requirement describes something the product has to do if it is to complete the work that it is intended to perform:
The product shall predict which road sections will freeze within the selected time parameters.
The reason for functional requirements is that once the business analyst has understood the necessary functionality of the product, he uses the functional requirements to explain to the developers what has to be built.