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 affected by character level and the Mana stat, as well as the class of the player. Maximum PP can be modified independently of its base attributes by certain effects, such as the mana battery corruption.
While magical class powers tend to require actual PP to work with, and mindcraft will only work with PP, regular spells can be cast using HP. Simply try to cast the spell as normal; if you do not have enough PP to cast it, the game will ask if you want to exhaust yourself to do it anyway. It will then drain HP equal to the amount of PP missing, divided by two, rounded up; in other words, each point of HP is worth 2 PP. In addition to this scaling cost, the player seems to be charged an additional 5% of their maximum hp in damage no matter how few HP are used to cast the spell (even a single point of HP loss will still incur a max-HP-dependent HP loss). Using this method to cast spells will however abuse the Mana stat, leading to possible reductions of the Mana stat and subsequently maximum PP score. This gives a heavy incentive to use only PP to cast spells except in emergencies, apart from the fact that losing HP to spellcasting might of course contribute to the PC's death as well (but often not as much as letting a particular monster live).
As of ADOM v. 1.2.0, HP casting also abuses the PC's Willpower and Toughness stats due to exhaustion; making it further undesirable for casting-orientated PCs to do. Casting from HP drains satiation at a rate of roughly six times the price in HP, so make sure you have enough satiation before huge loss in HP due to bookcasting "wish", for example.