In the referenced text, the comparison Angelou is making with the metaphors of bed as cooling board and blanket as winding sheet is "burial preparations for after death"
A metaphor is the practice of comparing tow unrelated nouns without using the words "like" or "as" with the aim of giving more strength to ones powers of description.
The lines are suggestive of the fact that the authors grandmother is grateful to be alive one more day.
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