Despite the terminology, in 5th edition a 'spell slot' is not a literal slot on your character sheet in which a spell is inserted. A spell slot is just a fungible resource expended at the time you cast a spell. (It's named 'spell slot' for the sake of nostalgia for 3rd edition D&D, when a wizard preparing a spell did involve assigning it a slot.)
Similar to the wizard and the druid, the cleric's Spellcasting class feature only says:
You prepare the list of cleric spells that are available for you to cast, choosing from the cleric spell list. When you do so, choose a number of cleric spells equal to your Wisdom modifier + your cleric level (minimum of one spell). The spells must be of a level for which you have spell slots.
The list of cleric spells that are available for you to cast, your prepared spells, is just that, a very simple unordered list. You don't assign each spell to any particular 'spell slot,' that last sentence only forbids you from choosing spells that are too high a level for you to possibly cast.
The Cleric table shows how many spell slots you have to cast your cleric spells of 1st level and higher. To cast one of these spells, you must expend a slot of the spell's level or higher. You regain all expended spell slots when you finish a long rest.
It says 'to cast one of these spells, you must expend a slot,' not 'to prepare one of these spells, you must...'
The legacy of the spell slot
Several site redesigns ago, wizards.com had a D&D Glossary summarizing terms from the then-recent edition, 3rd edition:
spell slot
The "space" in a spellcaster's mind dedicated to holding a spell of a particular spell level. A spellcaster has enough spell slots to accommodate an entire day's allotment of spells. Spellcasters who must prepare their spells in advance generally fill their spell slots during the preparation period, though a few slots can be left open for spells prepared later in the day. A spellcaster can always opt to fill a higher-level spell slot with a lower-level spell, if desired.
The term was newly invented for the 3e Player's Handbook (2000). At the time, figuring out which spells you can cast and how many times you can cast each of them required consulting:
- the table for Ability Modifiers and Bonus Spells on page 8,
- the generic rules for a class's Spells per Day on page 23,
- the character class table for your specific class,
- and the generic 'Casting A Spell' rules on page 148.
On page 148 it explains:
Choosing a Spell
First choose which spell to cast. A cleric, druid, experienced paladin,
experienced ranger, or wizard selects from among spells prepared
earlier in the day and not yet cast (see Preparing Wizard Spells, page
154, and Preparing Divine Spells, page 156). A bard or sorcerer can
select any spell he knows, provided the character is capable of
casting spells of that level or higher.
Once you’ve cast a prepared spell, you can’t cast it again until you
prepare it again. (If you’ve prepared multiple copies of a single spell,
you can cast each copy once.) If you’re a bard or sorcerer, casting a
spell counts against your daily limit for spells of that level, but you
can cast the same spell again if you haven’t reached your limit.
Spell Slots: The various character class tables in Chapter 3:
Classes show how many spells of each level a character can cast per
day. You always have the option to fill a higher-level spell slot with a
lower-level spell.
