A foreign currency (FX) pair is a quotation for two different currencies - i.e. the amount you would pay for one currency for a unit of another currency. For example, when someone is quoted 0.9 for the EUR/USD FX pair, it means that they can exchange €1 EUR and receive $0.9 USD. Foreign currency pairs are typically traded via the forex market - the world’s largest and most liquid asset class.
Within the foreign exchange market, currency unit prices are shown as currency pairs. The first currency within a currency pair quotation (e.g. ‘EUR’ in EUR/USD) is considered the “base currency”, with the second part of the quotation being the “quote currency”.