Basically it means that any changes and improvements you make to the source of Phoenix should be returned to the community as source.

However you don't have to publish any of your games source as MPL only the modified Phoenix source and you have to provide a note that you use Phoenix somewhere in a readme/credits etc.

The reasoning behind the MPL license is basically that the community (I so far) have spent a lot of time writing and maintaining the library and are providing it for free. Thus any changes made even by a commercial game should be returned as a "return favor".

I feel that the MPL licence is quite open as you basically can do what you want with it, if you changes anything in the Phoenix code let me know what you did (for my part you dont have to publish the changes to the world) so i can improve on the coming versions.