I think this requires a background in Aristotle's Metaphysics. In fact, this is the reason why many communities skip the first chapters of the Rambam's Code, and don't bother at all with the Guide to the Perplexed or with Rabbeinu Bachya ibn Paquda's Chovos haLvavos. As our surrounding culture has moved on from the Aristotelian worldview, these explanations aren't particularly relevant to us. And, in the eyes of some, involve spending more time learning Aristotle than is wise to do.
Since I am not among that "eyes of some", to take the complicated and try to put it into an oversimplified nutshell:
According to Artisotle's Physics, action starts with an intellect. The intellect moves an object by imparting impetus to it. Since no one had separated out the concept of friction (including air drag), there was no law of conservation of impetus; it’s not just another word for momentum. So, eventually impetus would run out, and the object would stop moving.
So, Aristotle's Metaphysics -- which literally means "Beyond Physics" (and I think referred to the fact that it was the volume after Aristotle's Physics) -- is all about intellects. They are the unseen that make the seen things happen.
The Rambam writes about angels Guide to the Perplexed 2:6, tr. Freidlander, which is flawed, but in the public domain):
We have already stated above that the angels are incorporeal. This agrees with the opinion of Aristotle: there is only this difference in the names employed — he uses the term ‘Intelligences,’ and we say instead ‘angels.'
So to the Rambam, angels are the intellects that mediate between Divine Will and things happening on earth. He writes in his code (Yesodei haTorah 2:5-6):
5: Since they [the angels] possess no body, what separates the form [of each] from the other? Their existence is not alike. Rather each one is below the level of the other and exists by virtue of its influence, one “above” the other. Everything exists by virtue of the influence of HQBH, and His Goodness. Solomon alluded to this idea in his wisdom, saying (Qoheles 5:7): “Because above the one who is high there is a watcher [and there are others higher than them].”
6: The expression “‘below’ the level of the other” does not refer to height in a spatial sense like, “He is sitting higher than his colleague”. For example, when speaking about two sages, one of whom is greater than the other, we say, “one is above the level of the other.” Similarly, a cause is referred to as “above” the effect [it produces].
So, to the Rambam, G-d had a Thought, which had a thought, and so on down a chain of ever more coarse intellects which eventually gets us to the highest level of angel (chayos haqodesh, in his system), down the various levels until you get to the 10th (ishim).
Bereishis Rabbah (10:6-7) says that every blade of grass has a mal'akh standing over it saying "Grow! Grow!" In the Rambam's worldview, this is because the mal'akh is the intellect that gives the impetus to the grass for the motions within its organs for it to grow.
And in his worldview, the reason why each angel can only have one task in this world is because two such processes would be two impetuses, and therefore by definition be two angels.
After the ishim, we get to the sefiros (spheres). The sefiros are so coarse that they are embodied. Not in the usual material, made of one or more of the 4 elements (fire, air, water or earth) but of a fifth essence, or "quintessence". The outermost sphere has the stars embedded in it. Then the spheres that have each of the planets in them, and these spheres could themselves have spheres embedded in them, which have spheres embedded in them, a whole complicated system to explain why planetary motion doesn't look like a simple rotation around the earth. (Google "epicycles".) And similarly the sun, and finally the moon. Now we get to normal matter. But what is important here is that the Rambam's sefiros are more like what we call "orbits" than the stars, planets, sun and moon. The sefirah moved the kokhav; the kokhav is a thing embedded in a transparent and quintessential seferah.
But we have orbits, because we live with an explanation that involves gravity and momentum. The path works because gravity and centripetal force are matched, and angular momentum is conserved. In Aristotle's description of the world, we're working with impetus. And impetus gets used up. So if the sefirah spins eternally, there must be an intellect involved replenishing the impetus.
At this point, I think we have the context to more fully understand the texts Alex cites in his answer.