Improved Two-Weapon Fighting[General]
Dex 17, Two-Weapon Fighting, base attack bonus +6.
In addition to the standard single extra attack you get with an off-hand weapon, you get a second attack with it, albeit at a -5 penalty. See the Two-Weapon Fighting special attack.
Without this feat, you can only get a single extra attack with an off-hand weapon.
A fighter may select Improved Two-Weapon Fighting as one of his fighter bonus feats.
A 6th-level ranger who has chosen the two-weapon combat style is treated as having Improved Two-Weapon Fighting, even if he does not have the prerequisites for it, but only when he is wearing light or no armor.