Well kiddo, do you know what darkness is? It's when everything is dark, and you can't see anything. Now, some people believe in something called Manichaeism, and they think there's a bad guy who lives in the darkness. They call him the Prince of Darkness.
They believe that the Prince of Darkness is always trying to do bad things and make people do bad things too. So, they say that there's a battle between the Prince of Darkness and a good guy named the Lord of Light.
Manichaeists also believe that all of the things we see in the world are a mix of good and bad. They call this mix of good and bad "dualism." They say that everything we do can either help the Lord of Light or help the Prince of Darkness.
So, the Prince of Darkness is like a bad guy who is always trying to do bad things, and we have to make sure we don't help him. That's why Manichaeists try to do good things to help the Lord of Light and not help the Prince of Darkness.