A more thorough explanation:

An open union is a type of labor union that allows non-members to participate in union activities and receive some of the benefits of union membership, such as collective bargaining and representation in disputes with employers. This is in contrast to a closed union, which only allows members to participate in union activities and receive benefits.

For example, a worker at a company that has a union may choose not to become a member of the union, but still be represented by the union in negotiations with the employer. They may also be able to attend union meetings and participate in other union activities.

Another example is a union that allows workers from different industries or professions to join, even if they are not directly employed by the same company. This can create a larger and more diverse membership base, which may have more bargaining power in negotiations with employers.

Open unions can be beneficial for both workers and employers. Workers who choose not to become members of the union can still receive some of the benefits of union representation, while employers may be able to negotiate more favorable terms with a larger and more diverse union membership.