When you use quick loot, you get the loot from creatures just by clicking on the corpse. Using this you can select the items you WANT to loot OR the items you DON'T WANT to loot.
Example: You kill a creature and the loot is 500 gold coins and 3 meats.
You can select to not loot meat so everytime you click to loot a creature that droped meat, you won't get it, no matter which creature was killed.
You can select to loot gold and everytime you kill a creature and click to quick loot, you will get the gold.
So when you add an item to "loot list" you are selecting if you want or don't want to loot this item when quick looting. If you open the "manage loot containers" window, you will find a list of every item you selected to loot or not and what option you choose "skipped loot" or "choose loot (or something like this I don't remember exactly)".
Personally, I prefer to use skipped loot to choose every item I don't want to loot.