It means you have two turbochargers on your engine.
andrew is correct... but to be correct there are 2 types of twin turbocharged setups: parallel and sequential in a paraller setup you have 2 turbos each using one half of the cylinders exhaust (i.e. in a V6 each turbo would be connected to 3 cylinders) this is done so that you can use smaller turbos which spool faster but can still make lots of boost. in a sequential setup you have a small turbo and a big turbo... both are hooked up to all the exhaust gases. the smaller one spools up quickly to get the boost started and to help spool the bigger turbo.. then the big turbo kicks in to deliver some really insane boost. it's a bitch to tune... but when done right it makes a ton of power.
As seen on a Porsche 959