Your base attack bonus (commonly referred to as BAB by the d20 community) is an attack roll bonus derived from character class and level. Base attack bonuses increase at different rates for different character classes. A character gains a second attack per round when his base attack bonus reaches +6, a third with a base attack bonus of +11 or higher, and a fourth with a base attack bonus of +16 or higher. Base attack bonuses gained from different classes, such as for a multiclass character, stack.
A character’s base attack bonus does not grant any more attacks after reaching +16, cannot be less than +0, and does not increase due to class levels after character level reaches 20th.
A minimum base attack bonus is required for certain feats.