This a beginner's question. There are fireign mastercard/visas right? Like, people in russia have MaterCards too that they use to pay for stuff with russion money on it?

Can someone in russia use a russian mastercard to buy something on my american website, and the money gets converted into US dollars by mastercard?

Yes, that's correct.

Specifically, you could be a U.S. business and have a foreign customer using a foreign-issued Visa or MasterCard.

They would typically purchase based upon USD from your Web site but on the settlement side of the process, their credit card statement would show the currency-converted amount in their local currency.