It just means employees of businesses that were able to continue operation during the pandemic lockdowns. Iirc, being a source of sustenance enabled fast food/takeout to remain open. Regardless of the nutritional value offered.
That’s how “essential worker” included both medical professionals and fast food. It is also how certain call centers were still crammed with people unless remote was made available. The phrase was seemingly used to help improve morale for those forced to work while benefits for the unemployed paid out more than their wages.
At the end they find out Frank owns it and they could have left at any time.