Base Currency
Notice that USD is on the left when is paired with some currencies but is on the right when paired with some currencies.
Any Currency that is on the left is called Base currency and it always equals to one. any currency that is on the right side is called Counter currency that the market will tell you value of Counter currency compared to base currency.
Examples
USDZAR
here 1 USD equals to 17.1233 ZAR. the left currency (base) always equals to 1 so platform will not write that, the platform will show the current value of counter currency in comparison to the base currency. 
EURUSD
here USD is on the right side (Counter Currency), then this means EUR is base and it equals to 1.
so 1 EUR equals to 1.16261 currently
Currency that is bigger is set as a base currency. value of base currency is always bigger that counter currency, thats why here USD is counter currency, it is because EUR is bigger than USD. GBP is also bigger than USD so you will observe that on some pairs USD is a counter Currency.
