Defense reduces the amount of Damage that the Enemy can do to your character. Some Leaders & characters when triggering their Adrenaline Rush, will provide a defense Bonus for a certain amount of turns.
Instead of attacking, you can touch and hold on a character and you will have the option to Defend by doing a swipe right.
Using Defend gives your character +50% Defense. If the character has Human Shield, it also forces human opponents to target the character who defended.
This can be a useful tactic to avoid fueling an enemies adrenaline rush bar.