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

When a spellcaster casts a spell using a slot that is of a higher level than the spell, the spell takes on the higher level for that casting. For instance, if a Wizard casts Magic Missile using a level 2 slot, that Magic Missile is level 2. Effectively, the spell expands to fill the slot it is put into.
Some spells, such as Magic Missile and Cure Wounds, have more powerful effects when cast at a higher level, as detailed in a spell's description.