404 is page not found, now if it appears immediately it means the web address you are trying to get doesn't exist ( check spellings usually) if it takes a time then appears its usually a problem with the network between you and the "Internet", check other websites to check your connection is working and then retry, if still broken again check the address is spelt correctly.
there are a whole set of error codes if you are at all interested here is a list and their meaning
List of HTTP status codes - Wikipedia, the free encyclopedia