The German income approach is a way to figure out how much money someone makes. Imagine you have a lemonade stand and sell cups of lemonade to people. You have to buy cups, lemons, sugar, and water to make lemonade. You also have to pay for the stand and maybe someone to help you sell the lemonade.
The German income approach works the same way. It looks at all the things someone needs to make money like their job, the things they need to do their job, and how much they have to pay for those things. Then it subtracts all the things they had to pay for from how much money they made to find out how much money they actually got to keep. This way, people can know how much they really earned after they paid for everything they needed to do their job.
So, in short, the German income approach is a way to figure out how much money someone makes after taking into account all the costs associated with their job.