AP: Attack Points
Represents your attack power. Your AP changes depending on the weapon you have equipped.

DP: Defense Points
Represents your defense power. Your DP changes depending on the shield and armor you have equipped.

CP: Charisma Points
The kind of items you can buy at shops change depending on your CP.

Sometimes you have to change your equipment to get past certain locations in the game.
To set up an item for use, select the item and press . The color of the item's name will change to indicate its ready for use.