The Pro Pack is an in-app purchase that requires real life money.
It can be purchased in the game itself. For this you need to go to the first menu -> Upgrade -> Purchase/Restore. It costs $0,99 (/0,89€).
The Pro Pack gives you access to more avatar features, such as headgears, as well as multiplayer combat features. In multiplayer combat, it grants you the ability to dual wield, as well as access to a huge cache of weapons to battle with.
Avatar skins: Edit
The Pro Pack will unlock following so called "body types" or options for customization: headgears, shoe coloring, accessoires like a kevlar vest or a chain magazine (they wont have any effect in the game)