AI overview
An HTTP 404 is an error message that appears when you try to access a webpage or file that doesn't exist or can't be found on the web server. It matters because it lets you know that the link you clicked or typed in leads nowhere, so you'll understand why the page won't load rather than wondering what went wrong.
AI-generated from the Wikipedia summary — may contain errors.
Article
English Wikipedia's 404 page
In HTTP, the 404 HTTP status code indicates that a web client (i.e. browser) was able to communicate with a server, but the server could not provide the requested resource. The server may not have the resource or it may not wish to disclose whether it has the resource. The code is often associated with response reason Not Found and is often referred to as page not found or file not found.