You can fight with a weapon in each hand, making extra attacks each round with the second weapon.
Prerequisites
Dexterity 17, Two-Weapon Fighting, base attack bonus +6.
Benefit
You gain a single iterative attack with an off-hand weapon, allowing you to attack twice with that weapon. This second attack is made at a -5 penalty. You must make a full attack to gain this second attack.
In addition, the penalty for fighting with two weapons is further reduced by 2. This removes the penalty while wielding a light weapon in your off hand and reduces the penalty for heavier weapons from -4 to -2.
The shield bonus to AC gained while wielding two weapons or a double weapon increases by one point to +2 (or to +3 when fighting defensively or using the total defense action).
Special
If you also possess the Multiweapon Fighting feat, you can make a single iterative attack with each off-hand weapon as part of a full attack.