Base Attack Bonus is one of the various stats that may increase when a player levels up. The base attack bonus is added to a player's other modifiers when making a weapon attack. Thus, having a larger base attack bonus allow a player to hit more often and to confirm criticals more often.
Base Attack Bonus by Class and LevelEdit
The classes in DDO are divided into three categories with respect to base attack bonus: classes that are good at fighting, classes that are fair at fighting, and classes that are poor at fighting. Depending on which level and which class a player is, he or she may gain 0 or +1 to base attack bonus at each level. In the case of multiclass characters, the base attack bonus from each of their classes is added together.
The classes that are good at fighting are the Fighter, the Barbarian, the Paladin, and the Ranger. The classes that are fair at fighting are the Rogue, the Monk, the Cleric, the Favored Soul, and the Bard. The classes that are poor at fighting are the Sorcerer, and the Wizard.
(Main article: Attack Chain)
In the Base Attack Bonus entry in the class pages, the base attack bonus is listed as a series of attack bonuses separated by slashes. For example, the Rogue page says that a Rogue's base attack bonus is +3/+3/+8/+13 at 5th level. This means that a level 5 Rogue has a four hit Attack Chain, and the 3rd and 4th hits of that chain have a higher attack bonus than normal. Refer to the individual class pages for details on each class' attack chains.