What does it mean that the quota is ‘backed up’ when filing for a green card after employment?

There are five preference categories in employment-based immigration. Each category has a limited number of green cards available per year. The most oversubscribed categories have more applicants than available slots, so earlier applicants are processed first. So once you begin your employment-based case, you receive a Priority Date, and your ‘turn’ comes once everyone who applied before you has been processed — the time it takes for your turn to come up is the waiting period. Whether your turn has come is announced monthly by the State Department in what’s called the Visa Bulletin (in Korean, referred to as the ‘영주권 문호’). So once your priority date category opens — meaning your priority date moves ahead of the announced cut-off date — you become eligible to file for your green card.