Average Cost is the "average" of the cost you set for an item in the Product Control Window.
If you go to Product Control and have an item, let's say it's a can of Soda for example. It's retail price is $1.50 but under cost you put 0.50 cents. Well if I understand it right, let's say a year ago your cost was set at .35 cents, now it's .50 cents but tomorrow it is going to go up to .60 cents and you religiously change it in the POS system as your cost is affected, then it will show you your "average" cost of the long run for that item.
One way to UPDATE this to current cost settings (which I prefer to do often as I adjust" is to go to Product Control and go to "Tools" and click "Set Avg Cost to Cost." This will reset your cost for items to exactly what you currently have for each product.
Hope that helps