Spell points, or just simply 'sp', are the points your character uses to cast spells or use skills. If you run out of them you have to either regenerate them, or refill them through other means, to start using spells and skills again.
Your characters maximum spell points is decided by your current intelligence. You inherently have 50 spell points at 1 int, and will gain an additional +8 for each point after that.
Maximum sp for a level 29 (regular) player with maximum intelligence modifers peaks at 362.