It simply means that nothing is blooming or if blooming is not putting out nectar, usually due to heat or lack of moisture. Usually in my locations, the fruit and dandelions and a shrub called caragana bloom pretty close to the same time. Then there is some years nothing at all blooming for two or three weeks until the sweet clover and alfalfa start blooming. That interval with nothing blooming is a dearth. Bees can build up hugely on the spring flow and feed everything coming in to larvae and starve to death while waiting for the main flow. After the killing frost in the fall, usually September, my main dearth lasts until mid APRIL.