This stands for International Alliance of Theatrical Stage Employees. It’s the largest labor union in the entertainment industry with about 115,000 members. While the DGA is the union for directors, WGA for writers, SAG-AFTRA for actors, IBT for teamsters, IATSE is a labor union for pretty much everyone else. This includes editors, production designers, DPs, camera operators, ADs, ACs, makeup artists, grips, gaffers, electricians, best boys, set builders, sound designers, sound mixers and many more workers.
Check out their website: http://www.iatse-intl.org/