Maat was the Egyptians' concept of truth, justice, or Right. The Egyptians believed they were only allowed into the afterlife if they had lived according to the rules of maat. Maat was made into a goddess who appeared in human form with a feather on her head.
In papyrus paintings of the Book of the Dead, the scales used in the weighing of the heart were shown with the feather of maat, or a statue of the goddess herself, on one side, with the dead soul's heart on the other side. The deceased hoped the magic power of the papyrus would ensure his heart balanced with maat, so he would be allowed into the afterlife.
By praying and offering to the gods, and by ruling justly, pharaohs made sure that maat was maintained throughout Egypt.