Generally, states charge Use Tax for any items that you buy out of state and bring into the state, or buy online, that you did not pay state Sales Tax on.
Depending on the state, it may ask you to estimate the dollar value of your un-taxed purchases, or it may offer a "safe harbor" amount based on your income. (That is, they have a formula that estimates the amount of use tax you owe based on your income, and how much of their income they guess people spend on un-taxed purchases.)
You can either choose the safe harbor amount, or enter an estimated value of un-taxed purchases, and the program will calculate the tax. If you think you had no un-taxed purchases you can claim zero.