A functional requirement describes an action that the product must take if it is to be useful to its operator—they arise from the work that your stakeholders need to do. Almost any action (calculate, inspect, publish, or most other active verbs) can be a functional requirement.
Functional requirements are things the product must do.
The product shall produce a schedule of all roads upon which ice is predicted to form within the given time parameter.
Link · 1. Some Fundamental Truths