A more thorough explanation:

Secondary picketing is a type of picketing where workers picket a business that is not their employer, but is connected to their employer in some way. For example, if workers at a factory go on strike, they may picket the offices of the company that supplies materials to the factory. This is called secondary picketing.

During a recent teacher's strike, the teachers picketed outside of the school district's headquarters. However, they also picketed outside of the offices of a company that provided textbooks to the school district. This is an example of secondary picketing.

Secondary picketing can be controversial because it can disrupt the operations of businesses that are not directly involved in the labor dispute. However, it can also be an effective way for workers to put pressure on their employer to meet their demands.