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!