Base Attack Bonus

The Base Attack Bonus or BAB is a number that represents your skill with a weapon. There are three BAB progressions. The best is +1 per level, which is reserved for the fighter classes (Fighter, Paladin, Ranger, and Barbarian). The next best is +3/4 per level which is used by the Bard, Cleric, and Rogue. The worst BAB progression +1/2 per level is used by the Sorcerer and the Wizard. Creature types grant for racial hit dice also one of the three BAB progressions.

The Base Attack Bonus determines the number of attacks you can make as every time your lowest attack reached a base of +6 you recieve another attack at +1. For example if you have a base attack of +10/+5 and then recieved another point of BAB then you would now have three attacks at +11/+6/+1. This means your first attack is far more likely to hit than your subsequent swings in an attack chain.

The fighter classes start the game at +1 BAB. Others start at +0 BAB.

For more see attack.