The "404" error is when something is not found on a website. It can be if the server is not responding or can't find the server. It is often used in websites that are malfunctioning, and other errors.
It is often indicated if the client is unable to communicate to the server and back. It is often seen in dead links and is known as one of the most infamous Internet errors. It has originated since the beginning of the internet, and is currently used today like most errors.
Other similar errors are 403 Forbidden, indicating that the user does not have the permission or their IP is blocked, and 502 Gateway Timeout, where the server is not responding. 508 if there was an infinite loop, and 511 for authentication required.