Most states are doing this now -- anything you buy out of state that you pay no sales tax on, that you would have paid sales tax on if you bought in-state, you are supposed to pay the use tax. What that is will depend on where you live and how much you buy online, and from what sites. For example, Amazon now charges New York sales tax on my purchases, and so do the few other stores I order from, so even though I have online orders, they are all already taxed so I put zero. On the other hand, a few years ago I bought a computer from an out of state company that did not charge sales tax and that was more than $100.
You really need to think about what you buy and where from and if those places already charge you sales tax or not. If you aren't sure, most states have a "safe harbor" amount based on your income -- they calculate an estimate based on your income and you can pay that with no questions asked. If you are sure you bought less out of state that was not taxed, then put the lower amount.
