Spell slots are the fuel you spend to cast spells.

For now, think of cantrips as being totally separate from spells.

All spellcasters, including warlocks, have a list of spells they can cast, through some combination of knowledge and preparation (the details vary by class). These spells all have levels — comprehend languages is a first-level spell, shatter is second-level, hold monster is fifth-level, etc.

Then (PHB, p.201):

When a character casts a spell, he or she expends a slot of that spell’s level or higher.

So to cast comprehend languages, you need to "spend" at least a first-level spell slot, but to cast hold monster you need to spend a fifth-level spell slot.

Most spellcasting classes — warlocks being the exception — have different numbers of slots of different levels. For example, a 9th-level wizard has four 1st-level slots, three each of second, third and fourth-level slots, and one fifth-level slot.

Warlocks are a bit different because they have a smaller number of slots, but all of the slots are of the same level. At first level, a warlock has one first-level slot, but at 9th level they have two fifth-level slots (but no slots of any lower levels).

When you cast a spell using a slot of a higher level than the spell — for example, casting shatter (a second-level spell) using a fifth-level slot — the spell may have increased effects. Shatter does extra damage when cast using a slot higher than second level. Other spells, such as comprehend languages have the same effects regardless of the spell slot used to cast them.

Once a warlock has spent their spell slots — at ninth level, once they have cast two spells — they can't cast any more spells (again, setting aside cantrips) until they've recovered their spell slots. Most spellcasters need a long rest (eight hours) to recover spell slots, but again warlocks are different, and recover all of their slots after a short rest (one hour).

To sum up, for a warlock:

• The spells you know dictate what you can use your spell slots for.
• You don't ever "use up" a known spell.
• You spend a spell slot to cast a spell you know. After that, the spell slot is gone, but you still know the spell.

Warlocks can swap known spells as they gain levels.

From the Known Spells of 1st Level and Higher section of the Warlock class description: (PHB, p. 107):

Additionally, when you gain a level in this class, you can choose one of the warlock spells you know and replace it with another spell from the warlock spell list, which also must be of a level for which you have spell slots.

For example when a warlock goes from 3rd to 4th level, their known spells goes from four to five (per the table on p. 107 of the PHB), so that warlock can add a new known spells of first or second level, and replace one of the first or second level spells they already know with another first or second level spell. This is how most warlocks will "upgrade" low-level known spells they don't need into more useful higher-level spells.

Individual rather than game-wide use is the intention of the Variant: Spell Points option. It's in the "Dungeon Master's Workshop" chapter, which has lots of advice and ideas for modifying the game to suit your table. Some of these are spelled out as game-wide. For example, under Combat Options, there are "Action Options", and the book says:

This section provides new action options for combat. They can be added as a group or individually to your game.

(That is, a group of all of these options, or just individual added actions. But, clearly, game-wide.)

On the other hand, this variant is under Creating New Character Options, and specifically "Modifying a Class". It says:

The classes in the Player’s Handbook capture a wide range of character archetypes, but your campaign world might have need of something more. The following section discusses ways to modify existing classes to better serve your game’s needs.

I think it's pretty clear that the intent here is for this option to be an example of a way to build a new, additional option to broaden the range of archetypes. Other sections here make it clear that this wouldn't need to even apply to all examples of a particular existing class. For example:

For example, you could decide that the clerics of a particular deity belong to an order that forbids the accumulation of material goods, other than magic items useful for their divine mission. Such clerics carry a staff, but they are forbidden from wearing armor or using weapons other than that staff.

So, come up with some reason why these druids are different. I might even consider going as far as to create a new druid circle that accesses magic in this way.

N.B. I have no experience with this option in play, and don't have particular knowledge or even opinion on whether it is unbalanced per se. I know answers which have that are preferred, but in this case, I think it's objectively clear that the option is meant to be a mix-in, not a game-wide replacement.