A more thorough explanation:

A Laidlaw vacancy is a term used in labor law that refers to a genuine job opening in an employer's workforce. This opening can result from the employer expanding its workforce, discharging a particular employee, or an employee resigning or leaving the employment.

Under the National Labor Relations Act, when a strike has been resolved, the employer must offer the job opening to the striking workers in order of seniority. This means that the most senior workers who were on strike have the first opportunity to fill the vacancy.

For example, if a company has ten employees and one of them resigns, creating a job opening, the employer must offer the job to the most senior worker who was on strike before offering it to any other employee. This ensures that the workers who went on strike are not penalized for their actions and have a fair chance to return to work.