A dodge bonus is a bonus applied to a character's Armor Class (AC). The most common source of a dodge bonus is feats, such as the appropriately-named Dodge (Feat), but also Mobility (Feat), and a variety of others. Other sources of dodge bonus include spells, class features, racial traits, etc.
Unlike most other bonus types, Dodge bonuses stack with themselves. That is, if a character has more than one source of Dodge bonuses, each of them can be added to the character's armor class.