This related to the size and location of the theater.
Broadway: a theater of greater than 500 seats in the theater district of New York.
Off-Broadway: a theater of between 99-500 seats, not necessarily in New York
Off-Off Broadway: pretty much anything else
Why I Looked It Up
I’ve noticed that most of the “Broadway theaters” aren’t actually on Broadway. They’re on numbered streets in the midtown around (42nd to maybe 45th).