When you take US appliances to Europe, or any of the many other places in the world where the 'mains' outlets supply 240 volts, you can't just plug your device into the 240 volts. It'll overheat, smoke, and die if you do.
You need some kind of a power "converter", that will change the 240V to the 120V that your device recognizes from back home, and is designed to work with.
If your device uses more than just a few watts of power to operate, then you'll need a converter that's based on a transformer. It'll be heavy and pretty bulky, but that's what it takes to do the job.
The transformer is a "step-down" unit. You put 240 volts into it, it steps that down, and 120 volts comes out of it.
