Maat is the Egyptian Goddess of truth. Her name meant truth in the ancient language of Egypt.
The feather on her head is a symbol of balance, truth, harmony and fairness...the spiritual idea of 'karma.'
The feather was used in the afterlife, to be weighed by the God Thoth against the heart of the departed.
If the heart was heavier than Maat's feather, it was fed to the monster, Ammit 'the soul eater' that was part lion, part crocodile and part hippopotamus.
If the departed's heart was balanced on the scales, they were allowed to continue their journey into the afterlife.
(This scene is from the temple of Maat and Hathor in Thebes.)
Maat's priests were involved in earthly judgements and sentences for crimes and misdeeds.
There is a temple of Maat at Karnack temple, built by Queen Hatshepsut, around 1460 BCE.
Maat is often shown on the walls of tombs, in the book of the dead spells, depicting the weighing of Her heart.
(painting 'Balanced?' by Ginger Strivelli)
Maat is the daughter of Ra, the Sun God, the God of Enlightenment.
She is the wife of Thoth, the God of Wisdom and learning...who performs the weighing of the heart of deprated souls against Maat's feather of truth.