Swarming is honey bee colonies' natural means of reproduction. By swarming a new honey bee colony is formed when the queen bee leaves the colony with a large group of worker bees. In the existing hive special queen cells are typically left behind so a new queen can emerge and continue that "mother" colony.
When the swarm leaves, they do not know where their new home will be. The swarm quickly finds a temporary location (in a tree, on a fence or mailbox, etc.) and sends out scout bees (perhaps several hundred) to look for a new home. This process can take less than an hour or last a number of days. While the swarm may look scary, typically they are very docile as they have no home or brood (developing bees) to protect. Just leave them alone and they will leave soon enough; or contact a local beekeeping club to come remove it.
For more information about swarming, you can read NCSBA Master Beekeeper Randall Austin's April 2019 article, Swarm season is here! Are you ready? (PDF format).
CGsBees uses bait hives at times to try to catch these swarms. Bait hives are made to be attractive to the scout bees by offering the size, smell, height from ground and entrance size honey bees seem to prefer in a permanent home. Once a colony chooses a bait hive as a permanent home, they are relocated to our apiary (bee yard)... typically within a week. This relocation, especially from urban environments, often offers a better location for the colony than inside the walls of a house or other enclosure that could be a nuisance for people.
Note: Special thanks to Suzy Spencer, NCSBA Master Beekeeper, for the use of her photo of a swarm under a bird house, at the top of the page.