A status code – or response code – is the HTTP response for the request of the page. These are always comprised of a three-digit number, where each band signifies a different response.

200 is a success, meaning the browser and the server can communicate and show content for the user.
Anything beginning with a 3 indicates a redirect, the most common being the 301 redirects which indicate that the requested URL has permanently moved and that a new page will be requested. Anything beginning with a 4 indicates that nothing is found, the most common of which is the 404, which usually servers an error page by the website explaining that the requested URL cannot be found.

Finally, anything with a 5 usually indicates a server issue. A 500 error will be served when the browser has made a valid request, i.e. the page exists, but the server cannot return the page.