I have never heard this term before. You learn something new all the time.

Far more veterans of recent campus activism have embraced the challenge of organizing the unorganized, on the new frontiers of the service sector. There, some national unions have been eager to train and deploy underground organizers—now more commonly known as “salts,” rather than as “colonizers,” with its older connotation of having a political agenda broader than just union building

One promoter of the inside approach is Peter Olney, who dropped out of Harvard (like Bill Gates, he notes) and later worked as an elevator operator and refrigeration mechanic at a unionized hospital in Boston. In the 1970s, Olney belonged to a “vanguard party” that encouraged its members to become coal miners, steel workers, and other kinds of industrial workers. In the 1980s, he moved to the west coast and became a full-time union organizer, ending up on the staff of the International Longshore and Warehouse Union (ILWU). The ILWU has a long history of support for progressive political causes, from its founding in the 1930s to the present day. But, like many other unions, its core membership has aged and shrunk, due to technological change, industry restructuring, and job elimination through attrition. After Olney became the union’s organizing director, he invoked the lessons of labor history when he called for “a broad-based plan that emphasizes ‘salting’ or planting organizers in key workplaces and industries” to help unions rebuild their lost strength.
Organizing For The Long Haul: Salting To The Rescue? | Talking Union