STC is a rating of how much sound is stopped by a partition, like a window or wall. It is a way to tell if one type of partion is better than another, without having to take someone's word for it. The higher the number, the better.
STC stands for Sound Transmission Class. It roughly indicates how many decibels can be stopped from passing through a partition. For a good explanation of the science, go to https://en.wikipedia.org/wiki/Sound_transmission_class