Okay, lets take this step by step. Here's one way to look at it:
The apes yawn and adore their fleas in the sun.
The parrots shriek as if they were on fire, or strut
Like cheap tarts to attract the stroller with the
Fatigued with indolence, tiger and lion
Lie still as the sun.
In this first stanza we are shown a group of very different animals. The apes are lounging around, the parrots are squawking, trying to get nuts from passers-by, and the lion and tiger are tired from being lazy. These are not animals that are naturally found together...parrots, lions, and tigers all come from different continents. And none of these habitats have "strollers." These facts let us know that the animals are probably part of some kind of zoo.
The boa-constrictor’s coil
Is a fossil. Cage after cage seems empty, or
Stinks of sleepers from the breathing straw.
It might be painted on a nursery wall
There is not a lot of "life" going on here (about as much as a painting)...the critters are lethargic and most seem to be sleeping and the snake is so still it might as well be dead stone! The only way you know they are alive is because the place has a stink to it.
But who runs like the rest past these arrives
At a cage where the crowd stands, stares, mesmerized,
As a child at a dream, at a jaguar hurrying enraged
Through prison darkness after the drills of his eyes On a short fierce fuse.
People aren't much interested in seeing these sluggy animals, either. They hurry past the cages indifferently. There is a crowd, though, around the jaguar enclosure. The jaguar is more interesting to watch because it is showing signs of its nature: it paces, angrily, "rebelling" against the cage. People are mesmerized by its vitality.
Not in boredom—
The eye satisfied to be blind in fire,
By the bang of blood in the brain deaf the ear—
He spins from the bars, but there’s no cage to him More than to the visionary his cell:
Despite the fact that the jaguar is physically in a cage, it is not in one mentally or "spiritually." It's nature cannot be held in check. The bars stop him, but they do not "cage" his spirt. The wildness is in his blood. He is no more tamed by the cage than a great human thinker's mind would be in jail.
His stride is wildernesses of freedom:
The world rolls under the long thrust of his heel.
Over the cage floor the horizons come.
His walk is the essence of wildness and freedom. The world is compelled by his powerful paws and the inner freedom of the Jaguar is not diluted.
So what does it mean? You would have to decide for yourself. It might be about the stoic nature of the jaguar, or about the spirit-breaking that goes on in zoos. It might be about humanity's interest in observing the wild (something largely driven out of ourselves) or about never giving up or giving in. Or maybe it is about all of these things. That's the beauty of poetry!
As in all of Hughes's animal-poems, in Jaguar too, he deals with the raw and savage power of the beasts--the hawk, the crow and the wolf. He himself associates these images of power to the tropes of the magical shaman; poetry, in his words, is a transformative mask. Though in most of his animal poems, Hughes's persona is located within the animal-self, in this poem, it seems to be an external observer, watching the Jaguar's movement from the imprisonment into a human world of civilization to its veritable liberation in a world of absolute power and energy, the ethico-political import of which remains equivocal, however.
In his typically anti-Romantic declination of an innocent animal world, Hughes sees the parrots as cheap tarts--a stark counter-anthropomorphism that sheds light on his fallen world of animals. The predominant setting of the poem seems to be a cage where the human will of mastery over the animal world is exhibitted. The animals in the cages are sterile, even the lion and the tiger are as still as the sun. The impotent animals in the cages look like painted prisons where an illusory image is at work. But the jaguar is introduced in the 3rd stanza as a counterpoint--a ball of fire, commanding massive spectatorial attention. the analogy between the gazing zoo-visitor and a child is reductive. In the fourth stanza, Hughes pays a Hughesian homage to the physicality of the jaguar's strength. it is seen as a Messiah, a leader, a visionary inspirer. As the opening line of the final stanza confirms, the jaguar is also seen as a poetic prototype--a visionary who makes the prison house his own cell of creativity, finding the much needed isolation in it. He destroys the encapsulation of the cage by making a metaphor out of it. The jaguar's cage is just a symbolic one where massive expanses of time and space merge and the cage-floor gets overshadowed by the infinity of the horizon.
The Jaguar thus shows an animal resistance to the human trope of mastery but Hughes, like a true visionary is able to see the underlying paradox of this subversively progressive move as the sheer savagery of the emancipatory power is laced with some irony, nevertheless.