What does E27 mean?
E27 stands for the type of the fitting of a lamp. E27 stands for a 27 mm diameter thread. "E" stands for Edison thread. So you always have to put an E27 light bulb in a fixture with an E27 fitting. E27 is the standard in England and most European countries.
The early light bulbs (remember them?) had a similar screw thread.
Be careful though, because there are also countries where E26 bulbs are the norm (for example in the United States).