A swarm is when the bees have run out of space in their hive and are planning to expand. If the bees are going to swarm the queen will lay an egg in a queen cup which the Nurse Bees will feed royal jelly so that the egg grows into a new queen. When a queen cup has an egg in it, it is called a Queen Cell
There are two types of queen cells, a supersedure cell and a swarm cell. The swarm cell is made when the bees are going to swarm. These cells have sixteen days total between egg and hatched queen, eight days when the cell is uncapped and eight days that the cell is capped.
You won’t want to open the hive to inspect it during the eight days that the queen cell is capped because if you do you might rip it open thus killing the queen developing inside. If you kill the queen you will either cost yourself money in buying a new queen or you will cost yourself a lot of time while the bees make another swarm cell
Swarms are very calm and aren’t likely to sting you because they have no home, no food, and no baby bees to guard. When the Swarm first leaves the hive they will go to a designated place (usually a tree branch, small shrub, or the side of a building) and there they will send out scouts to look for a new hive location. This is your time to catch the swarm, don’t panic, they usually stay there for around an hour so you have plenty of time.
First get a cardboard box to put the bees in, this is lighter than your wooden equipment so it’s more easily carried to higher places. If you can shake the thing that the bees are on hold the box under the bees and shake them into the box, if you can’t shake the thing the bees are on then use a Bee Brush to sweep the bees into the box.
You may need to repeat these steps s few times but once the bees seem to be staying in the box you can then dump the bees into the hive you want them in.
Keep an eye on the swarm for a while, they are going to be growing rapidly and are likely to outgrow the hive you have them in.