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

The Base Attack Bonus also determines the number of attacks you can make: in DDO you gain your second attack at +1 BAB, your third attack at +5 BAB, and your fourth attack at +10. The 3rd attack gets a +5 bonus to hit, and the 4th attack gets a +10 bonus to hit and double the threat range.

For instance, a 5th level fighter has three attacks with BAB +5/+5/+10, meaning the third attack in the attack chain is more likely to hit than the first two.