The exact line is "Life, like a dome of many-colour'd glass/Stains the white radiance of Eternity..." This is a clear example of a simile, a figure of speech in which one thing is compared to another of a different kind using the words "like," or "as." Similes are often used to emphasize a particular characteristic of a person or thing.

In "Adonais," Shelley is drawing attention to what was unique and special about Keats. Life consists of many different colored fragments, whereas eternity, where the recently deceased Keats now resides, is a bright, shining light, pure and unsullied. The implication here is that Keats was too gentle for this mortal world, and that he has now finally ascended to his rightful place in eternity, a place guaranteed by his immortal poetry. There is also a Platonic element to Shelley's tribute in that what is ideal, whether it's truth, beauty, or great works of poetry, never changes; it will endure forever. At the same time, mortal life, this "dome of many colour'd glass" is ever-changing, a poor copy of what is ideal, and so will continue to stain "the white radiance of eternity."