A pardlike Spirit beautiful and swift— A Love in desolation mask'd—a Power Girt round with weakness—it can scarce uplift The weight of the superincumbent hour; It is a dying lamp, a falling shower, A breaking billow; even whilst we speak Is it not broken? On the withering flower The killing sun smiles brightly: on a cheek The life can burn in blood, even while the heart may break.
Some leopard-like "Spirit" is also present (vocab note: a "pard" is a leopard). This Spirit appears weak and "in desolation mask'd" (mournful), but it is secretly full of love. He is, in fact, "girt" (belted) with weakness so heavy that it makes it hard for him to make it through the day or deal with his responsibilities (something that's "incumbent" is a duty). That weakness is probably from his grief.
The speaker uses a list of metaphorshere, from "dying lamp" to "breaking billow" (clouds). The grief sucks the life out of the frail man.
But, because of his love for the youth, the man doesn't perish. Shelley offers up another metaphor to explain why. Just like how the sun shines brightly on a flower that is wilting, or how someone can blush even while their heart is breaking, this man can be both suffering from grief and still showing signs of his love.