It appears to be an informal catergorisation of monsters—and possibly more.
Much of this answer comes from the content of this answer to a different question, which has been rephrased to be appropriate to this question.
Current examples of the term ‘kind of creature’ (or similar) include:
- Antipathy/sympathy (PHB p. 214):
a kind of intelligent creature, such as red dragons, goblins, or vampires
- Locate creature (PHB p. 256):
creature of a specific kind (such as a human or a unicorn)
- The Protector special purpose of a sentient magic item (DMG p. 216):
a particular race or kind of creature, such as elves or druids
- Wand of Orcus (DMG p. 227):
any kind of undead, not just skeletons and zombies
All emphasis mine.
From these examples, we can deduce the following:
Since ‘intelligent’ is pretty clearly a description of a creature, as opposed to part of ‘intelligent creature’ as a distinct term from ‘creature’, it follows that red dragons, goblins, and vampires and each a kind of creature.
Since ‘undead’ is a creature type, skeletons and zombies should therefore, under a less specific label, each be a kind of creature.
Similarly, the phasing of ‘creature of a specific kind’ suggests the same, if not a similar, meaning: thereofre, humans and unicorns are each a kind of creature (albeit potentially more specific).
However, at this point, we run into a problem: up until the last bullet point, every example mentioned only includes names which match closely to monster names—the series of red dragons of different ages; goblin; vampire; unicorn—but now, we have a much broader term: human. There is currently no monster entry with the name ‘Human’, but this example could still apply to a category of monsters. However, there are at least 46 which fit this description, not including specific adventure NPCs, so this would be a lot less sound a presumption—and this is despite the fact that this is supposed to be a ‘specific’ kind of creature!
We have a different problem with ‘druid’—which can reasonably be taken to be a ‘kind of creature’, since ‘elf’ (a race) is included in the pair of examples comprising both a ‘race’ and ‘kind’ of creature—whereby ‘druid’ can either refer to a monster of the same name or a creature with levels in the character class, which also potentially expands ‘kind of creature’ to refer to player characters.
However, no matter how we interpret the terms, it is clear that every listed example could solely apply to named monster descriptions. For example, ‘unicorn’ has a unique listing that it could refer to. We then have one possible consistent interpretation:
‘Kind of creature’ may refer to a category of monsters.
In any interpretation, we must have the following restriction, in response to the question about how this term interacts with a character’s options when casting the spell, in order to remain consistent with the examples listed so far:
A goblin target may not be turned into another creature which is a goblin.
This is necessary because ‘goblin’ is listed as a ‘kind of (intelligent) creature’. One could reasonably use this example to extrapolate that the target of a true polymorph spell may not be turned into a creature of the same race.
Aside from that, not much is clear.
The most general qualification to the descriptions listed in the various examples must unify race, class (if any), and whatever property being a zombie or skeleton could be.
For the purposes of differences to other transformations, they do not have the restrictions listed, while that allowed by true polymorph does.
The spells and features listed lack this restriction, with the only restrictions listed being CR or creature type. Thus, while the shapechange spell would be able to turn a goblin caster into the form of its goblin ally which has a lower level or CR, the true polymorph spell would not.