Spell slots in Dungeons and Dragons are similar to other RPG systems where the player has a certain number of “spare” spell slots that they can use to cast spells when they are not in combat or when they need to conserve their magic. The number of spell slots a player has is determined by their level.
In order to cast a spell, a player must have the correct spell slot for the spell they are trying to cast. For example, a player must have a spell slot for the fireball spell if they want to cast it.
The player can check their spell list to see what spells are available in their current spell slot.
If a player does not have the correct spell slot for the spell they are trying to cast, they can use one of their “spare” spell slots. If the player does not have any spare spell slots, they can’t cast any spells and will have to wait until they have some free spell slots.
How do Spell Slots Work in Dungeons and Dragons?.