Spell Point, or sometimes Mana Point, is unit of magical power, that is used to measure a hero's magical capability or the cost of casting a spell. A hero has spell points ten times his knowledge, and he uses those points to cast spells. Every spell has its casting cost, which means the number of spell points that will be subtracted from his mana reserves when the spell is cast. Having a school of magic secondary skill reduces the cost of all spells from that school by 1 point per level of the spell.
Normally heroes replenish one spell point per day. However, this can be enhanced with Mysticism secondary skill or with certain artifacts like Mystic Orb of Mana. A hero can replenish all spell points by starting a day from a town with Mage Guild. Another way to replenish all spell points is to visit an adventure map location called Magic Well. Additionally, a Magic Spring/Mana Vortex doesn't only replenish all spell points, but also doubles them.