Yes, if you want to require buyers to enter a code to claim the discount, enter that code in the Discount Code field. If you don't want to limit how many orders can be placed with that discount, just leave the Max. Use Count blank (or 0, same thing).
You would only leave the Discount Code field blank if you want a discount to be applied automatically, whenever an order meets the Min. Item Qty. and/or Min. Item Total threshold(s) you set for the discount, without requiring buyers to enter a code at all.