USD/CAD is the pairing of the US dollar against the Canadian dollar.
It shows you how many Canadian dollars you would need to purchase one US dollar. The first currency in the pair is the base currency – so in this case it would be USD. CAD is the second currency, so it is the quote currency.
For example, if the price for USD/CAD is at 1.28, this means it would cost $1.28CAD to purchase $1USD