The first currency in a currency pair (the one on the left side from the slash "/ ") is called Base Currency. Base currency always has a value of 1. The second currency in the currency pair (the one on the right side from the slash "/") is called counter currency or Quote Currency.
For example: EUR/USD
· EUR – base currency with value of 1.
· USD – quote (or counter) currency
Think of the base currency as the “root”. When you buy, for example EUR/USD, it means that you are buying the base currency (EUR) and simultaneously selling the quote currency (USD).