"Shut up" is a direct command with a meaning similar to "be quiet"', but which is commonly perceived as an angrier and more forceful demand to stop making noise or otherwise communicating. The phrase is probably a shortened form of "shut your mouth up", and its use is generally considered impolite.
More forceful forms of the phrase may be constructed by the infixation of modifiers, including "shut the hell up" and "shut the fuck up". In shut the heck up, heck is substituted for more aggressive modifiers. In instant messenger communications, these are in turn often abbreviated to STHU and STFU, respectively. Similar phrases include "hush" and "shush" or "hush up" and "shush up" (which are generally less aggressive). Another common variation is "shut your mouth", sometimes substituting "mouth" with another word conveying similar meaning, such as head, face, teeth, trap, yap, chops, crunch, cake-hole (in places including the UK and New Zealand), pie-hole (in the United States), or, more archaically, gob. Another variation, shut it, substitutes "it" for the mouth, leaving the thing to be shut to be understood by implication.