In 5e, spell slots are divided into three categories: cantrips, spells and martial maneuvers. A player has a certain number of cantrips at their disposal, which they may use once per turn.
Spells are cast using a spell slot and each spell requires a different amount of slots, with the most common being 1. Martial maneuvers use up an entire slot and can only be used once per turn.
So, if you have 4 spell slots available, you can cast 2 cantrips, 1 spell or 1 martial maneuver. If you have 3 spell slots available, you can cast 1 cantrip, 1 spell or 1 martial maneuver.
And finally, if you have 2 spell slots available, you can only cast 1 cantrip or martial maneuver.
The conclusion to this article is that in 5e, spell slots are very important and players will need to be careful not to waste them.