A 404 error, often referred to as "Not Found," is a standard status code that a server utilizes to communicate to a client's web browser that it cannot locate the webpage or file that the client has requested. This error message is a common response on the World Wide Web, and it is employed when a server encounters a request it cannot process.

The 404 error is not indicative of a problem with the client's computer or network. Instead, it signifies that the server has been unable to find the requested resource. There are several potential reasons for this, including the webpage or file being moved, deleted, or not existing in the first place.

When a client's web browser encounters a 404 error, it typically displays a message to the user indicating that the requested page or file was not found. This message may vary depending on the browser being used, but it generally includes a statement such as "404 Not Found." The browser then provides the user with options to either navigate to a different page or to refresh the current page.