Let's use the example of a microwave. There are many ways we, the actors or end-users, want to make use of the microwave, including heating up leftovers, boiling water for tea, defrosting frozen food, cooking a meal. Each one of these is a ‘use’ of the microwave and can be described in a use case. If we want to boil water for tea, we tell the microwave what we want done (our request to boil water). The microwave boils the water and notifies us when it's done.