You plug in the power supply to a 110v/220v source and it will output somewhere between 12v and 20V.

You can connect battery chargers, tire warmers, some soldering irons to the power supply. You can do this with multiple devices at the same time. Usually the connection is made with 4mm banana plugs connectors.

If you get a high power / high current power supply it can outlast your chargers or other accessories. It means that the devices can be smaller because they don't need to include a power supply circuit.