What does Hester cast off? Why?

Hester, in a moment of passion, says, "Let us not look back." She then undoes the scarlet letter and tosses it from her, watching it land only a few feet from the stream which would have carried it away. In line with this forest imagery, Hawthorne compares Hester's passion to the movement of a brook's water and its seeming sadness (a metaphor that has recurred throughout the novel). In this chapter she makes the decision to follow the brook deeper into the wilderness. In the woods, she is invigorated, brought to a new sense of life, so much so that she lets her hair down and throws away the scarlet letter.