Ply refers to the number of strands a yarn is composed of. The term ply is misleading. Some people assume that the more plys a yarn has, the thicker it is. Not true. Some bulky yarns have only 2 plys and some multi-ply yarns are as thin as baby/fingering weight. It is better to look for yarn in terms of weight such as baby, sport, worsted, bulky, super-bulky. These refer to the thickness. This is also how most yarns are labeled.