Patrimonialism is when someone in charge acts as if they own everything and everyone. Imagine if you have a toy box that you share with your friends, but one friend always takes all the toys and nobody else can play with them. That friend is being patrimonial.
In a bigger world, patrimonialism is when leaders of a country or organization treat everything like it belongs to them, even though it really belongs to everyone. They might take all the money or resources for themselves and their friends, and not care about anyone or anything else.
Patrimonialism is unfair because everyone should have a fair share of things, including toys, money, and resources. It can also cause problems because when one person or group has all the power, it can be hard for others to have a say in what happens.