The "404 Not Found" error is an HTTP status code that indicates the server could not find the requested resource. This typically occurs when a user tries to access a webpage that has been moved, deleted, or never existed. It serves as a way for the server to communicate that the specific URL entered is not available. When a user encounters a 404 error, they may see a message stating "404 Not Found" along with suggestions to return to the homepage or check the URL for errors. This error is common on the internet and can happen on any website, including popular platforms like Google or Wikipedia.