Three short lines capture one small moment: an elderly cat bending over his water dish. The speaker notices his sharp shoulder bones, the kind of thing you see when an animal has grown thin with age. Then comes the surprise. Instead of leaving him as a frail pet, the speaker calls him "a lion drinking / from a white clay bowl."
The comparison lifts the cat without hiding his age. The bones are still there, but so is a sense of dignity and wildness. The plain white bowl against the image of a lion makes the scene feel both ordinary and grand, like a household moment with something ancient in it.
The speaker seems tender and respectful, and perhaps a little sad, since the thinness hints at time passing. Yet the mood is mostly quiet admiration. The stepped, indented lines slow the reading, as if we are watching the cat drink, patiently and without hurry.