LORT stands for “League of Resident Theatres”. Many people who work in theatre think it stands for “League Of Regional Theatres”, but that is just a terrible myth.
LORT is a service organization. Its members are theatres all over the United States; it has about 80-90 members. All of them are not for profit professional theatres. In laymen’s terms that means that these theatres have mission statements focused around improving the cultural life of the cities they operate in—the “professional” bit means that they pay their actors, directors and designers according to one of the many union contracts that exist in theatre.
For example, Actor's Equity Association, (often called “Equity”) might say “you need to pay actors at least ___ when they work longer than ___” Since artists are notoriously poor not everyone follows these rules, but the LORT theatres always follow these rules.