Spell Slot
A limited supply of magical energy; casting most spells uses one up until you rest.
Spell slots are how many spells a caster can cast before they need to rest. Casting a spell uses a slot; resting gets them back. Slots come in levels: stronger spells need higher-level slots.
In Fantasy RPG
Fantasy RPG: cantrips are free. Other spells cast in live play use a 1st-level slot; a long rest restores every slot.