Selecting this shows you the current terms and allows you to change the terms of the sale. This includes the:
Discount Amount: This is the discount the customer receives if he pays within a certain date.
Discount Date: This is the date up to when the customer's discount is good.
Discount Terms: This displays the discount, how many days the discount is good for, and the number of days from the date of sale when payment is due.
If you apply a sales order to this sales/invoice, this information comes from the sales order.