You understand how to use Shortbows proficiently in combat and thus do not take a -4 penalty on attack rolls when using one.
Without the feat:
When using a weapon with which you are not proficient, you take a -4 penalty on attack rolls and an icon appears in the uppper right hand corner of the screen informing you that you not proficient with your equipment.
Special Notes:
Barbarians, Dark Hunters, Dragon Lords, Fighters, Paladins, and Rangers are automatically proficient with all martial weapons. They will gain this feat, and all subfeats (weapons types) automatically.
You can gain Martial Weapon Proficiency multiple times. Each time you take the feat, it applies to a new type of weapon. Alternatively you may multi-class with a class that is proficient with all martial weapons, and thus gain this feat and all sub-feats (weapon types) automatically.
