What does "Purchase Refund" mean?

You can get purchase refunds for two reasons:

1. If you're buying a commodity, and the auction house found someone had posted a cheaper stack up there compared to what you had looked like paying.

2. Someone else purchased the item you wanted to purchase, so the auction house refunded the amount for the items that you did not receive.

Basically, these are due to trades made between the time you saw the price of whatever you bought in the AH and when you clicked the buyout button. Since the AH does not automatically refresh, there could be a significant difference between what you can see and what's available (though you can get a different "That item is already bought" error depending on the timing).