Unit margin is the marginal profit for selling one more unit. It doesn't include the (large) fixed costs of, for example, your personnel or your office real estate costs or R&D or legal. It's basically the bill of materials for your item plus the costs for transporting the item from where you made it/bought it to where you hand it off to the customer, any taxes on that particular item (like import duties), etc.

Unit margin is an important number to account for and work to maximize, basically because you assume that as the number of units you sell increase, your fixed costs become less and less relevant to your overall company profit. But unit margin is always higher than overall company profit margin.

EDIT: Also, obviously, unit margin varies by sku. The iPhone 5s and the iPad Air are probably the highest-margin items that Apple sells (some of their Macs may compete). Things like iPods and iPhone 5cs are likely lower-margin.