He is a portion of the loveliness Which once he made more lovely: he doth bear His part, while the one Spirit's plastic stress Sweeps through the dull dense world, compelling there All new successions to the forms they wear; Torturing th' unwilling dross that checks its flight To its own likeness, as each mass may bear; And bursting in its beauty and its might From trees and beasts and men into the Heaven's light.
Now, Adonais-Keats is a part of the loveliness of the "Spirit," a word which the speaker uses to represent the entire universe, similarly to the way he used "Power" and "Nature" in earlier stanzas.
This force may be Shelley's concept of God. Whatever this force is, it contains everything, including Keats.
The speaker thinks that when we die we become one with the force. (Too bad he was a century and a half too early to see Star Wars.)
This force is stretchable, like plastic, and covers the whole world, which he describes as "dull" and "dense."
Compared to the afterlife, apparently it is pretty lame.
In the world, this force changes forms, making "dross" (rubbish) things into itself. It does this by filling these things (like trees, beasts, and men) with itself until they are ready to burst with "its beauty and its might."