In the Buying Unit of Measure field, you'll enter the item's buying unit of measure. This is the unit of measure by which the item is packaged when you purchase it. This unit of measure can be printed on purchases.
In the Selling Unit of Measure field, enter the item's selling unit of measure. This is the unit of measure by which this item is packaged when you sell it. This unit of measure can be printed on sales.
Some typical units of measure are Each, Pair, Case, Gross, Hour and Day. For example, assume one of your items is a lamp set that's always sold or purchased in pairs; this item could be assigned a unit of measure of Pair.