When a user interacts with an HTML form, the browser sends an HTTP request to the server. The server processes this request and sends back an HTTP response. This HTTP response contains data that the browser can use to update the webpage or carry out other actions.

In JavaScript, we can handle these HTTP responses using the Fetch API. The Fetch API returns a Promise that resolves to the Response object. This object represents the response to the request and includes methods to parse the response body into useful formats, like JSON or text.

Here's an example of using the Fetch API:

fetch('https://api.example.com/data', {
  method: 'POST',
  body: JSON.stringify({ key: 'value' }),
})
  .then((response) => {
    if (!response.ok) {
      throw new Error('Network response was not ok');
    }
    return response.json();
  })
  .then((data) => console.log(data))
  .catch((error) => console.log('Error:', error));

In this example, we make a POST request to 'https://api.example.com/data' with a JSON payload. We then parse the response as JSON and log it to the console. If the response is not successful (i.e., the status is not in the range 200-299), an error is thrown.

HTTP responses include status codes, which are three-digit numbers that provide information about the outcome of the HTTP request. For example, a 200 status code indicates a successful request, while a 404 status code means the requested resource could not be found. It's important to handle these status codes properly in your JavaScript code to ensure a smooth user experience.