A public sector undertaking is like playing with toys, but instead of toys, it's the government that controls things like businesses and companies.
Imagine the government has a big toy box full of companies. Some of these companies make toys, some make shoes, and some make cars. But instead of playing with these toys, the government actually owns them, and it is responsible for making them work well.
The people who are in charge of these companies are chosen by the government, and they make decisions based on what will be best for the company and for the people who use its products or services.
Now, when you go to the store to buy something, you might choose a toy or a snack made by one of these companies owned by the government. They might have a special logo on the product that tells you that it's part of a public sector undertaking.
So, in summary, a public sector undertaking is like a toy box full of companies that the government owns and runs to make sure that they work well and provide things that people need.