The phrase "union bosses" refers to the leaders or representatives of labor unions who make important decisions and negotiate on behalf of the union members. Full definition
Don't get me wrong — I am not implying that teacher union bosses don't care about children. (californiapolicycenter.org)
Local council leaders and trade union bosses are in touch with moderate MPs encouraging them to return to the fold and help improve the party's fortunes. (politicshome.com)