A more thorough explanation:

Secondary activity is a term used in labor law to describe a union's action of picketing or boycotting a neutral party in order to put economic pressure on them to stop doing business with the employer who is the primary subject of the labor dispute.

For example, if a union is in a dispute with a company over wages and working conditions, they may picket or boycott a supplier or customer of that company in order to put pressure on them to stop doing business with the company. This is considered a secondary activity because the supplier or customer is not directly involved in the labor dispute.

However, secondary activities are forbidden by the Labor-Management Relations Act, which means that unions are not allowed to engage in these types of actions. The act states that secondary activities are illegal and can result in legal action being taken against the union.

Overall, secondary activity is a controversial issue in labor law, as it involves balancing the rights of workers to protest and the rights of businesses to conduct their operations without interference.