The prefix is a country code with the first two letters of a patent number.
The prefix indicates the country were the patent was filed or granted. For example, when a patent is filed in the USA, the prefix will be US, when filed in China, CN, in Europe, EP and so on.
The complete list of prefixes can be found here: http://ep.espacenet.com/help?topic=countrycodes&method=handleHelpTopic&locale=en_ep