The main concept introduced by React in user interface design and implementation is the component concept. A user interface is an aggregate of components, and the whole React application is an aggregate of components. We will now see in more detail what components are from a design point of view.
From a design point of view, we can say that a component is a part of the user interface, having a specific role. A hierarchy of components is often called a component tree.
Consider a form in a web page. It can be treated as a component, since it has a specific role: to collect data and send it to the server. Also, a textbox inside the form can be considered a component. It has a specific role: to collect a single piece of data that will be sent to the server. So, a component may contain other components. And this is what usually happens: a user interface is a hierarchy of components, where some components contain other components.
Keep this concept in mind, since it will be useful to implement efficient and reusable user interfaces.