Sites are the number of unique IP addresses/hostnames that made requests to the server. Care should be taken when using this metric for anything other than that. Many users can appear to come from a single site, and they can also appear to come from many IP addresses, so it should be used simply as a rough gauge as to the number of visitors to your server.

A “site” is a remote machine that makes requests to your server, and is based on the remote machines IP Address/Hostname.