An item can be a common product that is sold such as a Sofa, or it could be a discount total or tax total that should be a line item on an invoice. You can set specific price levels for items that allow you to invoice more easily, as well as specify specific accounts that the items will affect when used in a transaction. Furthermore, items can be used to set specific inventory reorder points, and show you current inventory levels, or items that are currently sold.
When used correctly these Items give you the ability to greatly expand the ability of Quickbooks across multiple markets, and when used correctly vastly improve your ability to be efficient inside the program. The next time you are hand typing something on a form inside Quickbooks, perhaps an item would be better used for that.