I say "SEE toh", as if it were in Spanish or Italian. You can pronounce it however you like. I know some people say "SITE oh" or "SIT oh".
As for what it means... SITO is "OTIS" backwards. SITO used to be called OTIS until a certain art school's lawyers got huffy and puffy and threatened to blow our house down. OTIS is an acronym for "Operative Term is Stimulate". SITO is often thought of as "Still I Think OTIS". There are other expansions of the acronym: "Subversive Internet Terrorist Organisation", "Something Invested Toward Oculars", make up your own.
The word "sito" happens to mean "site" (as in web site) in Italian.