This command lets you drop two items that will give you one item that you will have to drop with another item to summon a zombie named BOB.
To use this command click the sign that says "Get Items" it will give you a diamond block glowstone and soul sand. Then drop the diamond block and the glowston on the ground together and you will get a command block. Finally drop the command block and the soul sand on the ground together and they will spawn BOB.