A Use Case (sometimes called a Command) is an explicitly defined way in which an application can be used.
Aside: Platform uses some generic CRUDS usecases. These aren't tied to a specific Domain Model (Entity) ie. a Post but rather have the entity and repo injected into them. This makes Use Cases significantly less well defined. A developer can no longer glance at the Use Case directory and see what actions are available. This might be something we can improve in future