HTTP codes
Forbidden
The page exists, the server just will not serve it without the right access.
Also called: forbidden, access denied, permission denied
403 is the status code for a refusal. The page is there, the server simply will not hand it over without the right access.
A 404 says nothing is here. A 403 says something is here, and it is not for you.
Where you meet it
Admin areas, staging sites and paid content answer with a 403 when you are not signed in as someone allowed to see them.