'mana points'
Mana is used for casting magic spells and performing some of the more magic-like skills. When mana drops to 0, you have no more resource to draw magic power from. Mana will regenerate over time, more quickly if SITting or RESTing, and even more quickly if SLEEPing. The HEALER can also help with very expedient mana recovery. See help hp mana move calc for more details.