During the spring months bees are outgrowing their winter home.
In order to not be overcrowded in their hive they make a new queen. The old queen will leave the hive with half the colony (bees) in search of a new home. During this time they will stop to rest, making a big clump together, looking like a big ball of bees hanging from trees branches, fences, cars etc. This is the time to call a beekeeper so they can remove them safely and place them in a new hive. If they move into a wall, meter box, etc. then it means that they have identified that to be a good place for a new home for them but that may not hold true for you. This kind of removal can be difficult depending on the the place they decided to move into to.