Some spells require reagents in order to cast them. Reagents are like "ingredients" — you have to have them in your inventory when you cast the spell, and they get used up when you cast it.
Reagents get used automatically when you cast the spell. You just have to have enough of the reagent in your top-level inventory (not in a bag), or in a reagent pouch.
You can figure out which reagents you need for a spell by looking at a spellbook for that spell or typing reagents <spellname>.