Borscht Belt
A series of mountain resorts in the Catskills of Upstate New York which catered largely to Jewish families. They were popular because they were within train distance from New York City at a time when air travel was new and expensive, and antisemitism was sill prevalent, which caused Jewish families to self-segregate to Jewish-friendly environments.
While they thrived from the 20s to the 60s, travel patterns and the social acceptance of Jews began to change, and the resorts went into decline, with most of them closing by the 70s.
Borscht is a beet soup, imported by the Jewish immigrants of Eastern Europe.
Why I Looked It Up
David Steinberg uses the term several times in his book Inside Comedy to refer to the resort/vacation circuit where a lot of famous stand-up comics of the 60s and 70s got their start.
Postscript
Added on
The phrase is mentioned several times in this article about a hotel fire: Catskills Hotel That Inspired “Dirty Dancing” Suffers Devastating Fire
Grossinger’s Catskill Resort Hotel was once the gem of the borscht belt, a bucolic mountain hideaway whose summertime community welcomed generations of New Yorkers and inspired the 1987 film “Dirty Dancing.”