As stated before, apps today use components to better structure the user interface. The idea is to break down the app to a set of encapsulated and reusable scoped components and avoid the primitive monolith approach.

Consequently, the user interface is built from a hierarchical component graph, comprising parent and child components. Often, this situation requires some sort of parent-child interaction control, such as the following:

• Parent needs to pass down input data to its child components
• Parent needs to respond when something occurs in a child component

Angular supports these two common scenarios using input and output.