It's a representation of the MediaWiki i18n message data. Most parameters are strings or integers, but some are "magic" arrays or hashes indicating special formatting. And you can even have nested i18n messages as parameters.

Unfortunately the exact format isn't very well specified at the moment. What we probably need to do is enhance the MessageSpecifier interface with those kinds of "magic" arrays and such, and possibly even define a serialization format in MessageSpecifier.