Durga always rides on a tiger, whom you see poking his head out on the left. Traditionally, Durga is powerful and she holds weapons in two of her hands, while two others offer blessings and protection.
Symbolically, she represents purity because she was created from the combined power of all the gods and goddesses. But her fierce nature suggests that those who wish to develop purity have to be prepared to change for the cause of developing purity.