A more thorough explanation:

A strikebreaker, also known as a scab or black-leg labor in British English, is a person who works under conditions that go against a union contract. This includes workers who cross a union picket line to replace a union worker during a strike.

• During the teacher's strike, the school district hired strikebreakers to continue teaching classes.
• When the factory workers went on strike, the company brought in strikebreakers to keep the production line running.

These examples illustrate how strikebreakers are used to replace union workers during a strike. This can cause tension and conflict between the striking workers and the strikebreakers, as the strikebreakers are seen as undermining the union's efforts to negotiate better working conditions.