Skill Points or "SP" are needed in order to perform skills.
Skill Points and Skill Point regeneration scale with player level. Some passive abilities lower the regeneration rate, for instance, the Assassin Class stealth togglable ability.
Some spells, such as Summon Guardian Angel, require skill points instead of magic points to use.
Some weapons and armor, such as the Horo-show Void Vanquisher, require skill points to activate their abilities.