Spell Slots

Spellcasting is taxing, so a spellcaster can cast only a limited number of level 1+ spells before resting. Spell slots are the main way a spellcaster's magical potential is represented. Each spellcasting class gives its members a limited number of spell slots of certain spell levels. For example, a level 3 Wizard has four level 1 spell slots and two level 2 slots.
When you cast a spell, you expend a slot of that spell's level or higher, effectively "filling" a slot with the spell. Imagine a spell slot is a groove of a certain size-small for a level 1 slot and larger for a higher-level spell. A level 1 spell fits into a slot of any size, but a level 2 spell fits only into a slot that's at least level 2. So when a level 3 Wizard casts Magic Missile, a level 1 spell, that Wizard spends one of four level 1 slots and has three remaining.
Finishing a Long Rest restores any expended spell slots.
Casting a Spell at a Higher Level