Answer:
A messenger from God who sets the boys free with the key of death and blissful life in heaven.
Explanation:
William Blake's poem "The Chimney Sweeper" from his "Songs of Innocence" talks about the issue of child labor. The main focus of the poem is a young chimney sweeper who had a dream about an angel and how that gave him the confidence to continue his 'work'.
The lines "And by came an Angel who had a bright key, And he opened the coffins & set them all free;" are from the 4th stanza. The young chimney sweeper, Tom, had seen an angel in his dream. And seeing that the angel brought a key to "set them all free" would help Tom happily continue working despite the gravity of being a chimney sweeper.
The angel's appearance is significant for it represents a messenger from God who would set the boys free and give them a blissful life in heaven.
