The PS4 MTX Key is a hardware modification for the PS4.
It allegedly allows groups of friends to share games between their PS4s with no limitation.
In other words, one of your friends buys a (digital) game on the PSN through a shared account, then the whole group can play the game on their own PS4.
The only difference between game-sharing and piracy is that initially, one of the users in the group needs to buy the original game, which can then be shared with a potentially unlimited group of friends.
Instruction can be seen here>>