Something that was mentioned before is aforementioned. Once you've written about something, it can then be referred to as aforementioned.
You already know that to mention something is to bring it up, so if you consider that afore sounds a little like before, you'll know that aforementioned is simply something that's been said previously. After your father has asked you to wash the dishes, do the laundry, and vacuum the rug, he might hand you five bucks upon completion of the aforementioned chores. This word shows up in writing more than speech. Writers often mention their own aforementioned ideas.