I am assuming that you want an explanation of the symbolism that occurs with this act of throwing the scarlet letter. Hester is experiencing an escape from her reality, the woods is a respite from the burden of the community. She, Pearl, and Dimmesdale are alone in the woods. As she takes down her hair, which is symbolic of Hester's letting herself relax, the sun shines down on them. She tosses the letter, and this is also a symbol of her feeling a freedom from her constant reminder of sin.
Yet, this is a mere respite, not a total transformation. She is lighthearted and beautiful for a time, yet the fact that the scarlet letter did not fall into the stream and get carried away, instead it gets caught in the shrubbery, symbolizes that Hester is not truly free of the burden of her sin.