HTTP is the Hypertext Transfer Protocol used on the World Wide Web to transfer data. It allows web servers to send information to browsers before any other output. Status codes are used in HTTP responses to indicate the status of a request, with 1xx codes indicating an informational response, 2xx codes indicating success, 3xx codes indicating redirection, 4xx codes indicating a client error, and 5xx codes indicating a server error. For example, a 100 status code means the server received the request headers and the client should send the request body.