"The sun comes forth, and many reptiles spawn; He sets, and each ephemeral insect then Is gather'd into death without a dawn, And the immortal stars awake again; So is it in the world of living men: A godlike mind soars forth, in its delight Making earth bare and veiling heaven, and when It sinks, the swarms that dimm'd or shar'd its light Leave to its kindred lamps the spirit's awful night."
Now, the sun is being personified as a "he." He rises, he sets, and meanwhile, creatures live and die. The speaker is all about the circle of life in this stanza.
But the stars never die; he calls them "immortal." Still, they sleep during the day and wake up at night. It's a never-ending pattern.
He says this pattern is the same for men's minds. A "godlike" mind goes forth like the sun, making other minds grow. But when that mind sets, like the sun (a.k.a. dies), it leaves all the other minds in the dark.
So who is this sun-like mind? Why, it's Keats, of course. The speaker says that after a man like Keats dies, our world is a little darker.