Power (watts) = Voltage (volts) x Current (amps)
50 watts at 220V = 0.227 amps ( 50 = 220 x 0.227 )
In very simple terms, you can think of it like this: Voltage is what the mains presents to the device ( the device must be rated to handle it ) and current is what is taken from the mains as the device operates. 13 amps is the most that can be taken before the fuse or breaker pops, and the device will take 0.2 amps, which is perfectly fine.
50 Watts is the two values multiplied together, and represents how much power is available for doing work. The higher the voltage the greater the power, and the higher the current, the greater the power.
The go to analogy for this is the flow of water:
You can have pressure (voltage) with no flow (current), such as when nothing is connected to the mains, and you can have flow (current) with a tiny amount of pressure (voltage). You can do a lot of work with high pressure, or a fast flow, and especially if you have both.