This is an exclamation that should be called if you are with someone and something happens that reminds you of a previous discussion you have had with that person. It is better when you shout it in a large group of people because then only one person will know what you are talking about and it is more fun.
It's basically just establishing that you and your friend both just thought of an inside joke between you.
Jackie: "Why were you so embarassed?"
Harold: "Because I was talking to my friends about how this girl had a mustache, and then i turned around and she was right behind me, she must have heard."
Stacie (to Jackie): "MattJaye!"
(Jackie laughs and understands because just last week Stacie and Jackie were talking about how Harold's ex-girlfriend had a mustache, what a funny coincidence this is.)