These three letters go in front of the name of a ship in the British navy. They stand for His Majesty’s Ship, or Her Majesty’s Ship when the monarch is a queen. So HMS Victory is a Royal Navy ship named Victory. In careful navy style you do not say “the HMS Victory” — the HMS already does the job of “the king’s ship” — though actually, plenty of ordinary speakers still put the in front, and you will hear both. Other countries use other letters (American navy ships take USS, for example). You will see HMS in history books, in the news when a British warship is mentioned, and in films about the navy. It is not used for fishing boats, ferries, or ships that merely sail under a British flag for trade. And the same three letters can mean other things in other rooms — even “hours, minutes, seconds” on a timer — but this entry is the ship’s title.