Before we get started
Since you asked for a simplified explanation, I will try my best to give simplified arguments with reference to answers where more precise arguments and primary sources are given.
The numbers in the brackets (e.g. 4:402) are a common academic way to refer to the Groundwork of the Metaphysics of Morals (and Kant's writings in general) according to the pagination of an old, German edition from around 1900. Do not let yourself be confused by them and other references and sources given. I included them to show more academically inclined users that I do not tell tales but can show where Kant did write what I state here (and where they can read about that if they are interested).
Nevertheless, I will probably give you more information here than you can chew at once. That is actually normal. Kant is not known for his simplicity. Do not shy away from asking for clarification in the comments section below or digesting it a bit before rereading or going on.
The question(s)
The question consists of three separate parts, which I will address separately:
- In which sense is the universal formula "identical" to the formula of humanity?
- Should the Formula of Humanity not be enough to determine a moral action?
- What are the criterions to define a member of Kant's "humanity"?
Identity of formulas
In a way, Kant's argument is quite simple and "analytical":
- I have shown that the Universal Formula is the epitome of a categorical imperative (Two times! In the first section deduced from the concept of "duty" (Ak. 4:397-402), in the second section deduced from the more general concept of "the will of a finite rational being" (4:412-421))
- In the same argument(s), I have shown that there can only be a single categorical imperative. (Disclaimer: Answer is mine)
- Therefore, all formulations of the categorical imperative have to be identical. (Disclaimer: Answer is mine)
- My argument leading to the formula of humanity ensures that it is a categorical imperative (esp. 4:426-429).
- Therefore, the Universal Formula and the Formula of Humanity are identical (from 1., 3., and 4.)
In short: If there can only be a single categorical imperative - and that is for sure - every variation on the formulations of this principle have to be identical (at least objectively).
As I wrote in the answer linked under point three, this does not mean that the formulations cannot have a "different feeling" to them (i.e. be subjectively different), just that they all are expressions of one and the same principle.
In a sense, the point he is making here is that "categorical imperative" does not mean a determined sentence with certain words in it. It is rather an idea or a set of sentences that essentially express the same thing - in this case: The core of what it means to act morally.
Is the Formula of Humanity not enough?
Well, yes and no. Yes, if you can intuitively understand what it means and apply it correctly. No, if that is not the case.
As written in the second answer linked, Kant does argue that the different variations are different ways to bring the categorical imperative close to intuition. Different people might prefer different ways (formulations) to approach and determine morality since they are more attuned to the different "feel" of it.
At the same time, Kant thinks that the "cleanest" way would be applying the Universal Formula directly since that is the one that most precisely expresses the categorical imperative without appeal to intuition and feeling (see 4:436-37).
Interestingly enough, he revises this view in a later book of his (Critique of Practical Reason). There, he uses a whole chapter to argue that we actually need something that makes morality approachable through intuition since we cannot really undestand and apply the "pure" categorical imperative (Schematism of practical reason).
What is humanity?
This one is a bit tricky. Kant is a bit ambiguous here. If we read really carefully, it becomes clear that "human" is just an example here: When he talks about "humanity", he really means "(finite) rational beings (like us)" (e.g. 4:414, 426, 428). Quite handy and solves all problems of understanding what this actually means, does it not? Well, not really. Let me try to explain in simple terms.
There are two things about finite rational beings he explicitly points out:
They have a will
A will is "the capacity to act according to [...] principles" (4:412). This means nothing more than that finite rational beings do not always act instinctively but also because of an understanding of what (usually) happens, can happen, or should happen. To infer particular actions from that, they need reason (reason is the faculty of principles and inferences)*, hence rational beings. Another way to put this (as e.g. Donald Davidson did): they can act according to reasons.
*source: Critique of Pure Reason A299–304/B355–361, A800–804/B828–832
They do not always follow their own reasoning
This is where "finite" comes into play: Yes, we humans (as an example) can (as a rule) act according to reasons and are most of the time "reasonable". But - truth hurts - we often enough act against our better judgement (4:412-13). In other words: We have instincts, desires, etc. that sometimes overwrite (or direct) our volition beyond reason.
So what about animals, mentally impaired, toddlers, etc.?
There is a huge discussion in the literature about that. Following the letter of what Kant writes, the outcome is quite harsh: While "rational animals" would count, many "humans", including the examples given, are not moral subjects since they "lack" a will in the narrow, above definition.
But what does this actually mean? Does that mean they are not "human"? Can we treat them like e.g. stones, or fish? Well, no. The thing is that Kant is speaking of a determination of what it means to act morally. And looking into our everyday lives, he has got a point: We do not deem and treat toddlers as immoral when they do something "bad", but rather as ignorant and protect them from being harmed by their actions based on our better judgement. And mentally impaired of a certain grade will have a legal guardian (just like children) not because they are not human but because they lack the ability to understand and rationally judge moral and legal implications of their actions.
Of course, this calls for discussions of the dangers of paternalism, i.e. where to make the cut. This is a topic of its own and actually a quite problematic one to decide for our courts.
