Salting is where labor union members hire on at a non-union business and assess workplace satisfaction and interest in organizing a union. US labor law prohibits unions from talking with workers in the workplace and salting is one of the few legal strategies left that allow union organizers to talk with workers.
Once the "salted" union employees determine that worker interest in organizing is sufficient, all workers at the business will be called at home (since they cannot be contacted at work) and a meeting will be called with the purpose of forming an Organizing Committee. A card drive will then commence, and once a majority of workers sign cards, the NLRB will be contacted and a date for an election set. Without salting, union organizers have no direct way to assess a workplace or to contact workers.