An agency shop is like a group of people who work together in an office. Everybody in the group gets paid money for their work, but some of the money they earn is taken out of their paycheck to pay for a special group that helps them all work together better. This group is called a union.
The union is made up of people who work to make sure everyone in the group is treated fairly and gets what they deserve. They ask for things like higher pay, better working conditions, and more benefits for everybody in the group.
If someone in the group doesn't want to be a part of the union, they don't have to join. But even if they don't join, they still have to pay some of their money to support the union. This is called an agency fee.
The agency fee is important because it helps the union to have enough money to do its job of making sure everyone in the group is treated fairly. Some people think it's not fair to have to pay money for something they don't want, but others think it's important to support the union so that everyone can benefit.