This refers to a combination of currencies that can be traded. Pairing is used to express the value of one currency against the other. In a pair, the first value is what you call as the base currency and the second that comes after the base is called the quote currency. The quotation of the currency pair is derived from the numeric expression of one currency against the value of the other it comes along with.
For example, EUR/USD 1.1104
In this case, EUR is considered as the base currency while the dollar is the quote currency. This expression denotes that 1 euro is equivalent to 1.1104 dollars.