The splendours of the firmament of time May be eclips'd, but are extinguish'd not; Like stars to their appointed height they climb, And death is a low mist which cannot blot The brightness it may veil. When lofty thought Lifts a young heart above its mortal lair, And love and life contend in it for what Shall be its earthly doom, the dead live there And move like winds of light on dark and stormy air.
Our speaker's talking about the pleasures of heaven ("firmament" is another word for "sky"), which may be overshadowed by earthy events for a while but can never be "extinguished," or made to end.
Death is just some lowly earth thing, like fog ("low mist"), which lingers close to the earth. It has nothing to do with the real good stuff, which happens in the afterlife.
What's more, when a young person has a "lofty"(important) thought, it takes their heart up above earthly things and into this higher realm.
While their heart is up there, personified love and life fight over it, trying to keep it from returning to earth.
Ultimately, this lofty thought makes it as though the young person has already died, since they are already kind of in heaven.
Shelley is equating lofty thoughts with self-preservation here. When someone thinks something profound, that thought lives on—even after they die. He considers Keats one of these lucky people who, because of their thoughts, never really die.