What does “Use Tax” in box 90 mean?

[AlexisH2]

Use tax is the tax paid to a state for buying items that you don't pay sales tax on. For example, I live in Massachusetts and drive up to New Hampshire, where there is no sales tax, and buy a car. When I bring it back to be used in Massachusetts, I am supposed to pay use tax to essentially make up for the fact that I didn't pay sales tax.