When you're adding a list of numbers, the subtotal is what you get after adding some (but not all) of them.
If you order a few pairs of socks online, your subtotal is the cost of the socks before tax or shipping costs are added. And if you're tallying up the number of doughnuts you've seen your brother eat in a week, you might get 10 as a subtotal before adding the final two he eats on Saturday. Subtotal adds the "under" or "almost" prefix sub- to total, from the Latin tōtus, "all, whole, entire."