You use this term whenever you see someone
crushing or
hitting on a guy. Usually in jokingly manner. You can either
shout it while the action is taking place (example 1) or in a way where you're telling them to make the first move (example 2).
Example 1:
"Omg guys look over there!"
"Is that
Jennie hitting on Kai?"
"(Shouts at Jennie from a distance)
GET YOUR MANS GIRL!"
Example 2
Jennie: "What am I even supposed to say if I go up to him?"
Lisa: "Just go, you're worrying way too much. Come on girl, go
get your mans"