An HTTP Status Code is a three-digit number returned by a web server to indicate the result of a client's request. These codes help users and developers understand whether a request was successful, encountered an error, or requires further action. They are categorized into five classes: informational, success, redirection, client error, and server error. Each class has specific codes, such as 200 for success, 404 for not found, and 500 for server error. By interpreting these codes, users can troubleshoot issues, while developers can improve their applications and ensure a better user experience.