Page 404 refers to the html page that a web server returns to us, when we access a url that does not exist or has ceased to exist, this is because it is the http error number for these cases.
notfound.org offers us to replace this response with images of missing children, so that when someone accesses a non-existent place on our website, this image appears and if they have seen that person, they can provide useful information to find them.