The Unit Inventory Value is the amount (in your local currency) something is worth from an inventory perspective. Generally speaking, it's the cost of the item when you bought it. In fact, the value is updated for you automatically by OfficeBooks when you buy an item using a purchase order. If you already have inventory of the item, OfficeBooks will use a weighted average calculation to adjust the Unit Inventory Value.
If you have 8 widgets, and the Unit Inventory Value is $3.00, your "Item total value" is $24.00 (8 x $3).
You can use the "Adjust Value" button to change the Unit Inventory Value manually. This should be done with care as changes in inventory value (typically) have tax implications for your business.
In the future, OfficeBooks will offer a set of depreciation tools to adjust values globally.
Careful: don't confuse the Unit Inventory Value with the sales price.