A successful request to the server results in a response, which is the message the server sends back to you, the client.
The response from the server will contain a three-digit status code. These codes can start with a 1, 2, 3, 4, or 5, and each set of codes means something different. (You can read the full list in http://en.wikipedia.org/wiki/List_of_HTTP_status_codes). They work like this:
1xx: You won't see these a lot. The server is saying, "Got it! I'm working on your request."
2xx: These mean "okay!" The server sends these when it's successfully responding to your request.
3xx: These mean "I can do what you want, but I have to do something else first." You might see this if a website has changed addresses and you're using the old one; the server might have to reroute the request before it can get you the resource you asked for.
4xx: These mean you probably made a mistake. The most famous is "404," meaning "file not found": you asked for a resource or web page that doesn't exist.
5xx: These mean the server goofed up and can't successfully respond to your request.