The base currency is the first currency appearing in a currency pair quotation. The second currency quoted expresses the number of units of that currency that are equal to one unit of the base currency. For example, if the EUR-USD is quoted at 1.25, EUR is the base currency, and USD is the quote currency.
It means, in this example, that one EUR is worth 1.25 USD. In most quoting conventions, USD is the base currency for emerging market currencies: USD-BRL, USD-TRY, USD-RBL, etc. In futures markets, which are mostly based in the United States, the base currency is always the foreign currency.