It's another way of saying "sup" or "what's up". It can also be used to express an action. To shup some birds would mean to get some bitches.
Shup mane. How's it going?
Guys let's just shup a party tonight.
When You try to talk but someone likes to always cut you off.
Me: Hi, how are we all today?
Mason: SHUP
*kid being annoying asf*
“Shup bruh ur so fucken annoying” - you
“Ok boomer” - kid