There is no definitive answer to this question, as the meaning of Ys can vary depending on context. In general, however, Ys is used to denote a small size, either physical or abstract. For example, a "youth size" shirt would be one that is meant to fit a child or teenager, while a "size Y" diamond ring would be one that is smaller in stature than average. In both cases, the use of Ys implies that the item in question is on the smaller end of the spectrum.