Not a ruler, an egg layer, at the heart of the colony's chemistry
A queen begins as a fertilised egg identical to any worker's. The difference is diet, larvae selected to become queens are fed royal jelly, a nutrient dense secretion, throughout their development, which triggers full ovarian development and a larger body size.
A young queen leaves the hive on one or several mating flights, mating in the air with multiple drones from other colonies, then stores their sperm for the rest of her life, using it to fertilise eggs as needed for years afterward.
During peak season a productive queen can lay more than 1,500 eggs a day, more than her own body weight in eggs, choosing whether to fertilise each egg, fertilised eggs become female workers or queens, unfertilised eggs become male drones.
The queen produces a blend of chemical signals, sometimes called queen substance, that suppresses ovary development in worker bees and signals her presence and health throughout the hive. A weakening or absent signal is one of the triggers that leads workers to begin rearing a replacement queen.
When a colony loses its queen, or she ages, workers select several young larvae to raise as queen candidates. The first to emerge typically kills her rival queen cells, or the two queens fight, so that the colony is left with a single queen once again.