USS goes in front of the name of an American navy ship to show that it belongs to the United States government: USS Enterprise, USS Missouri, USS Cole. It is a title, not a word you use on its own — you would not say 'a USS' — and it is used only for warships that have been officially put into service, so a ship being built is not called USS yet. In careful writing you say 'the USS Nimitz' or just 'USS Nimitz'; naval people usually drop the 'the'. Other countries have their own version of this: British ships take HMS. Science fiction has made the letters famous far beyond the navy, since the starship in Star Trek is the USS Enterprise. Say the letters one by one: U-S-S.