A character can choose to Fight Defensively in combat, diminishing their offensive capabilities in order to focus on defense. While fighting defensively, a character gains a +2 Dodge bonus to Armor Class while suffering a -4 penalty on melee Attack Rolls and Combat Maneuver checks. This Dodge bonus to Armor Class is lost any time you lose your Dexterity Bonus to Armor Class (when Flat-Footed, held, prone, paralyzed, sleeping, etc.)
If you have three ranks in the Mobility skill, this Dodge bonus to Armor Class increases to +3. Note that three ranks does not mean a +3 Mobility skill bonus, it means you must have spent three skill points on Mobility.
You can activate the Fight Defensively ability in the Abilities tab (the middle tab) above the action bar, and it can be toggled on and off at will.