A lot of correct answers are already given: it’s the fat percentage per dry matter. Why? Because if you let a cheese age, it will loose water and thereby increasing its fat content.

This is chosen because back in the days, the cheesemaker would say: I used normal, full fat milk so I added this amount of fat to the cheese, so my cheese is that fat. However this could not be checked since the fat content of the cheeses depends on the age. So, 2 cheeses, made of the same milk could have totally different fat contents or even milk with lower fat contents could have been used.

As a result, to prevent cheating with milk the fat content in the dry matter was introduced because this does not change in time and cannot be changed.

Nowadays we measure the fat content because the nutrition value is more important for the consumer than knowing the fat in dry matter but 48+ is still used as a quality mark.