The feather which represents Ma'at is most commonly known/recognized as the object against which the heart (ib) of the deceased is weighed in the "Judgement scene" (aka Spell/Chapter 125) from the Ancient Egyptian Book of the Dead.

But what is Ma'at? Ma'at is one of, if not the single most important concept of Ancient Egyptian cultural and theological belief. Like many ancient Egyptian concepts, it is multilayered and complex. It is order, truth and equilibrium. It is correct thought and action. It feeds and sustains the gods ('Netjeru'). Indeed there is no higher offering. It maintains the correct functioning of both The Cosmos and human community. It is the air which the blessed dead breathe. Aside from being represented by this feather symbol, Ma'at can also be represented as a goddess, easily identifiable by the presence of the feather on her head.

My favourite quote about Ma'at comes from Erik Hornung (Idea into Image, p. 143): “As a guiding principle maat is a point of orientation or a standard of measurement; it is the “ought” against which the “is” of life is ruthlessly and repeatedly measured and almost always found lacking.”