In Emily Dickinson's poem "Hope is the thing," Dickinson provides a very simplistic metaphor. The comparison made speaks to the idea that the Hope is a bird which sings in the soul.
That being said, the storm, which is seen in line six, is used as a metaphor as well. Here, instead of comparing hope to a bird, the storm is compared to the things (unnamed) which could attack the hope of a person.
The importance of the use of the storm shows that as long as one sings with their soul, there is no possible way that a storm (or tumultuous things which come against a person) could harm them. The bird, described as the one who "kept so many warm," cannot be harmed by the storm as long as the bird keeps singing (or having hope).
Therefore, the storm, simplistically, represents all of the things which could happen in one's life that would take away their hope.