Basically means "hurry up, idiot." Comes from when you're playing a video game with a non-gamer and you have to tell them to press A on the controller to advance to the next screen, especially when your patience is running low.
Video Game Situation
Non-gamer: Duhhr.. now what do I do?
Gamer: PRESS A! Geez.
Aquata: Press A and get on the bus before it leaves!
Alana: Spencer, press A and get out of my room!