It’s where when you battle, you have a chance of the monster you are battling to possess an item (This will give the monster buffs such as strength boost). This can be anywhere from a MYTHICAL INFINITY GAUNTLET to a common boot. When you defeat a monster with an item, you obtain the item and can bind it to a monster of your choosing to give it battle buffs.
And finally, if you get all the items you get an achievement (like when it says “Wow you have all the bromon” when you get the bromon).