A more thorough explanation:

Definition: A scab is a worker who goes against the terms of a union contract, especially by crossing a picket line during a strike to replace a union worker. In British English, a scab is also known as a black-leg laborer.

• During the strike, the company hired scabs to keep the factory running.
• The union members were angry at the scabs who crossed the picket line to work.
• John was labeled a scab by his former colleagues after he took a job during the strike.

These examples illustrate how a scab is a worker who goes against the union's agreement and works during a strike. They are often seen as betraying their fellow workers and can face backlash from the union members.