Shut up

"Shut up" is a command meaning to "be quiet". However, it is more forceful, likely used to tell someone to stop making noise or talking nonsense. It is also a short form of "shut your mouth up". This use is sometimes rude and disrespectful.
Variations.
There are more forceful commands, such as shut the hell up and shut the fuck up. In messages, they are sometimes abbreviated to STHU and STFU, respectively. Similar phrases like "hush", "shush", or "shush up" are similar to the "shut up" command.