Maat was both the goddess and the concept of truth, justice and order (everything that was opposite to evil and chaos). On the highest level this concept referred to the cosmic order, created out of chaos at the beginning of time by the creator god. This order had to be maintained continuously, as the forces of chaos were constantly lurking. This was one of the most important tasks of the king.
He did so by offering to the gods to make them stay in Egypt, which would guarantee that order would remain in the country; if they were to leave, this would bring Egypt back into a state of chaos. A typical offer in this context consisted of a statuette of the goddess, being offered to her lord, for example to the sun god Re (Maat was considered to be his daughter) or to Ptah (who is often portrayed standing on a dais in the shape of the hieroglyph for Maat). Both gods can be called “Lord of Maat”, as can others.
The king also issued laws, based upon Maat and aiming to maintain Maat in the country.
In the judgement of the dead Maat was present in one of the scales of the balance on which the heart of the deceased was weighed. Sometimes she was represented there by a feather, which is also a hieroglyph for her name.
Maat is usually depicted as a woman with a feather on her head.