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"
A phrase someone says when a man does something polite or charming. It means to get a respectful boyfriend like the guy you are referencing is to his girlfriend.
Jenna: Hey Charlie, can you go get me some more water?
Charlie: Sure (gets up to go get Jenna water)
Jenna: Can you also grab the ketchup?
Charlie: Yeah (grabs ketchup and water and brings them to Jenna)
Alice: Get you a mans like this!