Regardless of how many spells a caster knows or prepares, he or she can cast only a limited number of spells before resting. Manipulating the fabric of magic and channeling its energy into even a simple spell is physically and mentally taxing, and higher level spells are even more so. Thus, each spellcasting class's description (except that of the warlock) includes a table showing how many spell slots of each spell level a character can use at each character level. For example, the 3rd-level wizard Umara has four 1st-level spell slots and two 2nd-level slots.
When a character casts a spell, he or she expends a slot of that spell's level or higher, effectively “filling” a slot with the spell. You can think of a spell slot as a groove of a certain size-small for a 1st-level slot, larger for a spell of higher level. A 1st-level spell fits into a slot of any size, but a 9th-level spell fits only in a 9th-level slot. So when Umara casts magic missile, a 1st-level spell, she spends one of her four 1st-level slots and has three remaining.
Finishing a long rest restores any expended spell slots.
Some characters and monsters have special abilities that let them cast spells without using spell slots. For example, a monk who follows the Way of the Four Elements, a warlock who chooses certain eldritch invocations, and a pit fiend from the Nine Hells can all cast spells in such a way.