Initially, Helen Keller was able to "imitate" words in sign language. This meant that she could make the necessary movements in the right circumstances. However, this does not mean that she was actually able to understand the meaning and the implications of what she was signing.
However, after a while, her mimicking became true understanding. She went from only being able to fake a movement to being able to actually communicate. This allowed her to gain an understanding of the world that surrounded her. It also allowed her to engage herself more deeply into her reality. It allowed her to go from being an outsider to being a participant in life.