PP is their physical strength. You could also say that its your blocking power too. Any attack made against you if blocked would subtract their damage from this. IF you have 0 left in your pp, remaining damage would go directly to your hp. Note: This would work on only successful blocks. For example, lets say something doing 50 damage to you hits you, but you block it physically. Your hp is 500 and your pp is 450. Your pp goes down to 400 and no damage goes to your hp. You attack during your attack turn, but he then attacks you for 500 damage and you block successfully. YOur pp goes down to 0 and the remaining 100 damage lowers your hp to 400. Its pretty simple actually. However, your physical attacks are related to this. Also note that the only attacks that do over that much damage needs to be trained and they are the ultimate attacks only.

HP is your total hit points. This is essential not to get to the 0 cause once your hp runs out, the fight is over and you are either dead or unconcious. However, you can think of hp as your physical endurance too. When you charge up, you get 10% of your hp to your pp and sp. For example, if your hp is 500, you gain 50 to your sp and pp, if your hp is 380, you gain 38, etc. You always round up for this. The best strategy is to keep the hp up as much as possible cause you will need it.

SP is the total amount of your ki. Any ki moves you use drain from this. There is NO exceptions. This is another form of blocking. Note though, barriers drain from the sp for cost, and any damage would be done to the sp before the hp. Same system as the pp above. SP is also used to determine how much your ki attacks do. You would want to keep it up as much as possible too.

MP is used to determine how much your magic attacks do. It is also used to use magic attacks. Any magic spell you use depletes from your mp and your mp attacks focus on your mp as well.