A more thorough explanation:

A closed shop is a type of workplace where you have to be a member of a specific union to work there. This means that if you want to work at a closed shop, you have to join the union first. Closed shops were allowed in the past, but they are now illegal in the United States because of the Taft-Hartley Act of 1947.

For example, let's say there is a factory that is a closed shop. If you want to work there, you have to be a member of the union that the factory is associated with. If you are not a member of the union, you cannot work there.

After closed shops were made illegal, some unions started advocating for union shops instead. In a union shop, you don't have to be a member of the union to work there, but you have to join the union within a certain number of days after you start working. If you don't join the union, you can be fired.

For example, let's say there is a restaurant that is a union shop. If you want to work there, you don't have to be a member of the union, but you have to join within 30 days of starting your job. If you don't join, you can be fired.

In 2018, the Supreme Court ruled that union shops are also illegal. This means that employers cannot require their employees to join a union or pay union dues as a condition of employment.