A rial is a unit of money: the standard cash of Saudi Arabia and of some other Arab countries, the way dollar, yen, or pound names a country’s money. You will also see the spelling riyal; English uses both, and they are not a spelling mistake. Banks, travel sites, and newspapers use the word in a dry, factual way — “priced in rials,” “the rial rose” — not as slang. People in those countries have their own everyday names for the notes in their pockets, but an English learner mainly needs the name of the unit. Iran has a rial too, and the name is the same kind of word, but that is a different country’s money, not the Arab-country sense this entry is for. Though actually, English is messy here: rial and riyal get used for related currencies, and you cannot tell from the letters alone which country’s money is meant, so the country name usually has to travel with the word.