Use Case diagrams illustrate the services a system provides and the actors which care about those services. A system, when viewed through the lens of a Use Case diagram, can be thought of as a tool which provides utility/value to someone. The Use Case diagram makes that value clear.
Uses Cases are a behavior of a system. Primary Actors are the people that invoke a Use Case. Secondary actors are anyone who is involved in the behavior.
from https://en.wikipedia.org/wiki/Use_case_diagram#/media/File:Use_case_restaurant_model.svg
