"Use" tax is when something bypassed sales taxes, so that agency wants their tax, anyway. Example: A construction contractor buys and installs kitchen cabinets for your remodel; they buy it tax exempt and charge you the sale tax. If they also install some in a spec home they are building, they are the end user, so they should not have bought these cabinets tax exempt. They now owe use tax on the cabinets installed in their own project. Or, you stock inventory including paper products, you buy them wholesale (for retail), and then you take some printer paper or toilet paper off the shelf to stock your office = that means you are the end user and you owe the sales taxes not paid at the time of purchase, for the materials you used instead of selling at retail, and instead of charging the sales tax at the time of the sale. A lot of agencies are pursuing tax on internet sales, since the favorable ruling for South Dakota, as Wayfair vs SD.

I'll bet if you put your cursor in the Use tax box and then click the Yellow ? button it will explain it to you.