Tourism is an export industry because foreign visitors who travel to a country purchase the "touristic experience" of that country. They pay for it with their money from their country.

That means, that e.g. a Frenchman buys with his money which he earned in France something (in this case the touristic experience = hotels, commodities, activities, car rentals etc. but also service etc.) which is manufactured in e.g. Spain. This can then be considered to be an export of those Spanish "touristic goods" to France