What does it mean to make up the difference in hotel rooms?

This situation generally exists in the case of online group purchase and hotel reservation, that is, when booking online, I was told that there was a room available, and when I checked in, I was told that the reserved room was full or other reasons, and then I was asked to make up a certain price and arrange a room with a higher grade than the reserved room. Anyway, it's just right to find a reason for you to add more money.