Controller
A controller is an asset that represents a player controller in game. It is used to easily bind controls to functionality.
A controller holds a series of input mappings which can be interpreted by each respective module to create gameplay behaviour. Each button can be simply bound to a regular unity input action to customize which button corresponds to which action.
For a concrete example, have a look at the Abilities input module.
Creating a new controller
Controllers are scriptable objects and to create one, you'll need to right click on the Project Panel and navigate to Create → Game Creator → Input → Controller.