The controller function in Angular Js is used to maintain, add, and delete application data. It attaches behaviors and states to HTML elements and scope objects. Multiple methods can be attached to the scope object within the controller. This might include event handlers that trigger an event in case of a mouse click, keyboard click, etc., and execute a method. The properties and methods are visible only to the current controller and its child elements. Complex objects can be attached, and nested controllers can also be created in which the child controller can access properties of the parent, but vice versa is not true. This is the significance of the controller in Angular Js.