When you purchase this key, you get to choose which of the 7 items you want.

In the server, you can choose one of these:
1 Pig Spawner
1 Zombie Spawner
1 Creeper Spawner
1 Skeleton Spawner
1 Blaze Spawner
1 Iron Golem Spawner
1 Breeze Spawner

Each spawner generates mob drops when you are near them.