Okay... This will need a bit of background explanation!
Maat was firstly a goddess. She was shown as a woman with an ostrich feather on her head. She represented law, truth, honesty, and balance.
The Egyptians did not believe in the afterlife in the way it is viewed now, as heaven or hell. Instead they believed that once you died, you then had to process through a series of challenges, aided by what is now known as The Book Of The Dead. This in reality was a series of spells or magic words to help the dead person pass through the numerous gates.
Once you completed these challenges, you came to the final test, known as the weighing of the heart. The dead person's heart was placed on a scale, against the feather of Maat. To pass on to the Filed Of Reeds, the heart had to weigh equal to the feather - the dead person had to show they had done maat in their life. (For those that are interested, here is a list of the things the heart had to proclaim to weigh equally.)
If a person had not upheld maat, their heart would be consumed by a monster, and they would die the 'death of deaths', and they would live no more.
It's important to know that to understand the role that the law of maat held within every day life. I find it fascinating and utterly wonderful that this culture, which survived for thousands of years, was governed by something that, in essence, boils down to leading a life of balance.