A large number of worker bees, drones, and usually the old queen that leaves the parent colony to establish a new colony.
Swarming is a honey bee colony’s natural means of reproduction. In the process of swarming, a single colony splits into two or more distinct colonies. Swarming is mainly a spring phenomenon, usually within a two- or three-week period depending on the locale, but occasional swarms can happen throughout the producing season. Secondary afterswarms, or cast swarms may happen. Cast swarms are usually smaller and are accompanied by a virgin queen. Sometimes a beehive will swarm in succession until it is almost totally depleted of workers. (wikipedia)
See video of a swarm leaving an observation hive
“Observation Hive Swarms, Queens Pipe, Capture Demonstration, Hiving That Swarm, Honey Bees & more” Note: queen leaves at 15:02-15:13