Stagger is the white line under the enemies health bar which slowly builds up as the battle goes on. Drax's initial Special Ability builds up stagger in a significant way. The other easy way to stagger enemies is with shooting with Star-Lord's charged shot. It takes some practice but what you'll want to do is double tap the right trigger and then hold on your second pull. The game will lock onto an enemy if you successfully pull it off an start charging.