Your mental reserves are measured by your Mana. This is chiefly the domain of those who practice the magical arts, which often require incredible feats of mental concentration. Thus, with each incantation you chant, or with each magical potion you brew, you will find your Mana decreasing accordingly. When it reaches zero your powers are rendered useless until such time as they are restored. The simplest way of recovering mana is by meditating - type MEDITATE, and the amount of mana restored will depend on how long before you break your meditation by typing another command.
You can MEDITATE at any time, whether you have mana to restore or not; whether you are at your maximum mana or not. If you break meditation prematurely you will find the act of shaking yourself free of the trance bears a slight mana cost. Be wary of the meditative state since you are more vulnerable to certain covert attacks while you remain in the trance.
Mana is used by all of the magical skills and by those physical skills which require intense concentration. Bear in mind than when you have been slain, it will be your mana which keeps your corpse from rotting away. Thus, a high mana will keep your corpse alive longer. Equally, a high mana will make you more resistant to magical attacks since your level of concentration will be that much greater.