Does it stand for anything?
"pssht"
I'd say that it's likely a variation on "psst".
http://www.dictionary.cambridge.org/define.asp?key=63815&dict=CALD
http://www.dictionary.cambridge.org/define.asp?key=63815&dict=CALD
I also found this, but it's making no sense:
http://www.urbandictionary.com/define.php?term=pssht
http://www.urbandictionary.com/define.php?term=pssht
So it looks like a thing that a handful of teenagers are into do at the moment. They do this gesture and make that sound and it probably is funny or even cool to insiders but I'll bet it just looks absurd to outsiders. It'll most likely die out in a couple of years.