Exchange rates always quote the number of one currency that is equal to 1 unit of the other currency. The currency being quoted against (the one which is 1 unit) is the base currency.
So, for example, if 1.2 $’s = 1 Euro, then euro is the base currency.
I do suggest that you watch my free lectures on this. The lectures are a complete free course for Paper F9 and cover everything needed to be able to pass the exam well.