A strikebreaker is just someone who is hired to work in place of a worker who is out on strike. Strikebreakers are typically used by employers to keep their businesses running when their regular employees are on strikes. Strikebreakers are also known pejoratively as “scabs” by people who support labor unions.
During a strike, the regular employees of a company refuse to work. They hope to force the employer to give in to their demands so that business can return to normal. However, sometimes employers will not want to give in to the strikers’ demands. They will feel that they can get by with other workers. A good example of this was what we saw early in the National Football League’s 2012-13 season. The NFL locked out the game officials and brought replacement officials in to officiate the games. Of course, the referees were not on strike (they were locked out) but the same principle applies. The idea was to bring in new workers so the employer could go on operating without meeting the demands of the employees.
So, strikebreakers are people who come in to work at a place where a strike is going on. If the company can do well enough with the strikebreakers working, the strikers will have to give up and come back to work.