Substats were secondary stats required to equip some equipment. For example, your character might have to meet a STR stat requirement and a DEX substat requirement in order to equip a particular piece of armor.
Substats were removed from most equipment in the 11/28/2012 update to increase the equipment options available to all characters. Now, most equipment will only have a single stat requirement. Keep your equipment’s stat requirements in mind when assigning your Ability Points (AP).
If you find an item that still has substat requirements, please post in the Bug/Glitch forums to bring this to our attention.