In React, state is an object that stores information about the component's current state or state changes. The state object is accessible only within the component itself, and can be updated using the setState() method. When the state of a component changes, React will rerender the component and any child components that depend the updated state.

Here is an example of how state can be defined and used in a React component:

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import React, { useState } from 'react';

function Counter() {
  const [count, setCount] = useState(0);

  function increment() {
    setCount(count + 1);
  }

  return (
    <div>
      <p>Count: {count}</p>
      <button onClick={increment}>Increment</button>
    </div>
  );
}

In this example, count is the state variable, which is initialized with a value of 0 using the useState() hook. The increment() function updates the state by calling setCount(), passing in the new count value. Finally, the component renders the count value within a paragraph element and a button that, when clicked, calls the increment() function to update the state.

In React, you can display the content in an <h2> element by using setState() inside a handleSubmit() function.

For example:

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class MyForm extends React.Component {
  constructor() {
    super();
    this.state = {
      message: ""
    };
  }
  handleSubmit(e) {
    e.preventDefault();
    this.setState({
      message: "Submission Successful!"
    });
  }
  render() {
    return (
      <form onSubmit={this.handleSubmit.bind(this)}>
      <input type="submit" value="Submit" />
      {this.state.message ? <h2>{this.state.message}</h2> : null}
      </form>
    );
  }
}

To use an API with React, you will need to make an HTTP request to the API server endpoint. This can be done using an HTTP library such as axios or the native fetch() API.

Once you have the data from the endpoint, you can use the setState react hook to bind the data to your component's state and render the information to the page.

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// In your component's constructor
constructor(props) {
  super(props);
  this.state = { apiData: [] };
}

// In the function that calls the API
fetchData() {
  Axios.get('http://example.com/api/endpoint')
    .then(response => {
      // Set state with the data returned from the endpoint
      this.setState({ apiData: response.data })
    });
}

// Render the data 
render() {
  return (
    {this.state.apiData.map(data => (
      <span>{data.name}</span>
    ))}
  )
}

Additionally, you can use the React Context API to store the fetched data and bind it to certain components in your application, making it available and reusable in different states of your application.

In React, a state management system is used to manage the state of components. It is a technique used to make the data and state of a React application easier to maintain. React provides a few methods that developers can use to manage this state, such as setState(), useState, and useReducer. For more information and examples, checkout the React docs on state and lifecycle.

You can change an HTML element text inside React using setState(). This can be done by calling the function and then setting the new value for HTML elements like h2 as shown in the example below.

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class App extends React.Component {
  constructor(props) {
    super(props);
    this.state = { 
        text: 'Hello world!'
    }
  }

  handleTextChange = () => {
      this.setState({text: 'This is my new text!'});
  }

  render() {
    return (
      <div>
        <h2>{this.state.text}</h2>
        <button onClick={this.handleTextChange}>Change text</button>
      </div>
    )
  }
}

You can read more about this use case in this article.