Selecting this shows you the current terms and allows you to change the terms of the purchase. This includes the following:
Discount Amount: the discount you receive if you pay within a certain date
Discount Date: the date up to which the vendor's discount is good
Displayed Terms: the discount, how many days the discount is good for, and the number of days from the date of purchase when payment is due.
If you apply this purchase invoice to a purchase order, the information comes from the purchase order.