You understand how to use all types of simple weapons in combat (see Table: Weapons for a list of simple weapons).
Benefit: You make attack rolls with simple weapons normally.
Normal: When using a weapon with which you are not proficient, you take a -4 penalty on attack rolls.
Special: All characters except for Druids, Monks and Wizards are automatically proficient with all simple weapons. They need not select this feat.
Source: Player’s Handbook (Page 100)