In React, render() is a lifecycle method that is used to render the component's user interface (UI) based on its current state and props. The render() method returns a description of what the UI should look like, using JSX syntax.

When a React component's state or props change, React will call the render() method again to update the UI with the new information. React will then compare the new UI with the previous UI, and only update the parts of the DOM that have changed, which can result in improved performance.

Here's an example of a simple React component that uses the render() method to display some text:

In this example, the render() method returns a JSX element that describes the UI for the component: a div element containing the text "Hello, world!". When this component is rendered, the render() method will be called to create the UI.