A more thorough explanation:

A captive audience is when an employer gives speeches to their employees that discourage them from joining a union or are against unions. These speeches may be given during work hours and on company property, but they cannot be coercive. The employer does not have to give the union a chance to respond to the speech during work hours on company property. However, there is an exception to this rule during the 24-hour "cooling off" period before a representation election. If the employer violates this rule, the representation election can be set aside.

One example of a captive audience is when an employer holds a mandatory meeting during work hours to talk about the downsides of joining a union. The employees are required to attend the meeting and cannot leave until it is over. Another example is when an employer sends out emails or posts signs around the workplace that discourage employees from joining a union.

These examples illustrate the definition of a captive audience because the employer is using their power and authority to influence their employees' opinions about unions. The employees are not given a fair chance to hear both sides of the argument, and the employer is taking advantage of their captive audience to push their own agenda.