It means you would be best to find an new adapter at the electronics shop that outputs the same DC voltage (12V DC 1.2A).
To do a little bit of explaining, just in case anyone was wondering. I may not get all the technical aspects right, but I will try to explain a little in plain English so that people can understand.
There are two main types of electricity that we deal with every day. There is AC and DC (Alternating Current and Direct Current). The big thing to remember here is that need to be converted in order to work, and any sort of conversion wastes energy, so the less you convert the better. Also, AC travels long distances well without disappearing into the ether so to speak, but can kill you fairly easily. DC works great over short distances only and won't kill you (in most cases) but just give you a bit of a shock.
The best way to think of electricity is to think of it in terms of a water pipe. If you do, most electrical terms are easy to understand. First there is Voltage. Think of this as how big around the pipe is. The bigger the voltage, the more electricity that can flow through a pipe. Second is Amperage (Amps), thing of this like water flowing through a pipe, the more Amps there are, the more electricity is going through the pipe. Third major one to think of is frequency, which is measured in Hertz (Hz). This only applies to Alternating Current and basically is how fast the current alternates. Hard to think of this in terms of water, but if the water were to suddenly go back up the pipe very quickly before going down, that is kind of the concept with frequency.
Less important, but still fun to think of is resistance and capacitance. Resistance simply is as it sounds, it is like a very narrow pipe that restricts the flow of electricity. Capacitance is the opposite in a way, but it is like a chamber or a bucket in the pipe that holds a certain amount of electricity and then releases it at a different rate then might have flowed into it.
To bring power into our houses, everyone uses AC (there was a big argument between Edison and Tesla about this, which I am sure you can find out about online if you are curious). The big problem is that different standards were used between the US and the UK. The first difference is that in the US the Voltage is 110 Volts and in the UK it 240 Volts. The second is that the frequency is different. In the US it is 60Hz and in the UK it is 50Hz. While most of this doesn't really make a difference to the average human being, it does when you attempt to plug something in. Basically for UK stuff, the pipe is too small and the device will never get enough electricity to function if you plugged it in the US. If you took a US device, expecting 110 Volts suddenly gets 240V... You guessed it... *KABOOM* and smoke and maybe fire... Oh what fun.
Now back to Amps briefly. This is how fast the electricity flows (or think of it as being sucked) down the pipe. In your house, you fuses/breakers are rated for a certain amount of Amps. This is because the more amps you suck down a wire, the hotter the wire gets. If you were to exceed the rated amount, you would end up with a fire in your walls if you sucked more then the wire could handle, sort of like bursting a pipe. Not good. Normally we don't worry about this, until you plug in those Christmas lights into a socket with 20 other things and *snap* the breaker goes.
Most electronic devices that we use in our houses these days though, run of DC, at least internally. Our batteries are all DC. Therefore you have to convert the AC to DC via a transformer. In computers and larger things like stereos, these are built in. Because we are changing things, because of the laws of physics, it means we lose some of the energy and it becomes heat. That is why there are fans on computers and the like to vent this. Now some things, like our portable electronics have the transformer separate. This means we don't have to lug it around with us, so our MP3 players, portable computer and the like often come with what is referred to as a "brick", but what it does is convert our household electricity from AC to DC as well as regulate the amount of Amps output in order to make sure the right amount of energy is going into the device.
So where is this all going you ask? Well, if you understand the numbers on the back of your devices, you can figure out if all you need is a plug adapter, a new AC/DC transformer, or if you are stuck with a step transformer (one that converts one voltage of AC to another). Most people focus on step transformers for a lot of stuff, when it is probably easier to get a new AC/DC transformer. On the other hand, a lot of things take multiple voltages in if they use DC internally (it keeps from having to make lots of different versions for the same stuff all over the world and all they have to do is send the right plug).
So in the case above, 120V AC 60Hz means this is standard US house voltage, but the charger outputs only 12V DC 1.2A (12 volts of direct current at 1.2 amps). So the bad news is, it doesn't take UK voltage (240V AC 50Hz). You could try and get an AC step transformer for this, but that would probably be a waste of money. Also you have to make sure that you get one that produces the right amount of Amps (1.2 in this case). If you powered other devices off it, you would look for a step transformer that produced an aggregate of your amps (so if you had two 1.2 Amp devices, you would need at least a 2.4 amp transformer). By the way, since things like hair dryers draw a huge amount of amps, you are much better off buying a new hair dryer in the UK.
Another option here is when you are in the UK, go to your local electronics shop (e.g. Maplin) and try and find an replacement AC to DC charger. What you need is something that takes UK voltage but outputs the same DC voltage (in this case 12 volts DC at 1.2 amps). Most of these replacements ones will have a variety of plug adapters for the DC end of the device, which are usually round little plugs and the AC side of it will have a standard UK socket plug.
As a side note, because of the 240 voltage in the UK is one of the main reasons of the rather larger socket compared to the US. Because the pipe is bigger, the wires need a little more insulation as well as they need to be much more difficult to get into with little fingers. That is why electrical standards are so high, like the plug sockets being fused as well as having an automatic shield that only lifts when the plug is inserted, etc...
Which brings me to another thing for US folks dealing with the UK, if something stops working, and you have checked the fuse/breaker box and nothing is wrong, don't automatically assume the device is shot. Check the plug and you will see a little fuse in there, check and maybe replace that before chucking out the device.
