A banknote is a piece of paper money — the kind in your wallet, not coins. A bank, or the government behind it, stands behind the paper so shops will take it. You can say a ten-dollar banknote. Though actually, in American English people most often say bill (a dollar bill) for the same thing; note is common in British English. Banknote sounds a little more careful than cash. Fake banknotes are copies made to cheat people. The word is only for paper money, not for the numbers in a phone banking app.