In ADOM, PP stands for power points. It reflects the amount magical energy currently residing within a character, waiting to be used to fuel magical spells or other abilities. It is primarily determined by character level, the Mana and Willpower attributes, and class. Maximum PP can be modified independently by certain effects, such as the mana battery corruption. When low on PP, it's possible to instead cast spells using HP.
The maximum amount of bonus (independent) PP you can get is 9999.