That's a swarm - get in touch with your local beekeepers group/organisation and someone will probably come and collect it.

Otherwise don't fret, the bees will do their thing. Scout bees will leave the swarm and look for a new home for the colony (the queen plus a third to a half of the original hive). They report back with details, and a collective decision is made on the best choice. The swarm will then leave. (Honeybee Democracy https://press.princeton.edu/books/hardcover/9780691147215/honeybee-democracy is a good read on this.)

Back home the rest of the colony have a new queen pupating. She'll eventually hatch out, go on a mating flight, and come back to start laying eggs.

Good things to do: call a bee keeper. Do this.

Bad things to do: throw rocks. Don't do this.

That is likely a new swarm looking for a home. When an established hive gets too crowded, they will create a new queen who will then fly away with a subset of the workers to establish a new hive.

If you want to make your local beekeeper's club very happy, call them and tell them where this swarm is! They will come and collect it.