A more thorough explanation:

A sympathy strike is a type of strike where union members who do not have a grievance against their own employer stop working to show support for another union that is involved in a labor dispute. This is done to put pressure on the employer to meet the demands of the striking union.

For example, if the workers at a factory are on strike, the workers at a nearby warehouse may also go on strike to show support for their cause, even if they do not have any issues with their own employer.

This type of strike is also known as a solidarity strike.