A more thorough explanation:

An agency shop is a type of workplace where employees are not required to be full members of a union, but the union still represents them. Both union members and non-union members have to pay regular union dues and initiation fees, even if they choose not to join the union. This is because it is assumed that any collective bargaining done by the union will benefit all employees, regardless of their union membership status.

For example, let's say there is a factory where some employees are members of a union and others are not. The union negotiates a new contract with the factory management that includes higher wages and better benefits for all employees. Even the non-union employees who did not participate in the negotiations will still benefit from the new contract because they work in the same factory and are covered by the same terms.

Another example could be a school district where teachers can choose whether or not to join the teachers' union. If the union negotiates a new contract that includes higher salaries and better working conditions, all teachers in the district, whether they are union members or not, will benefit from these improvements.