What does Ply mean?
A measure of the number of strands twisted together to generate yarn.
Heddels explains Ply
The ply refers to the number of strands (called singles) twisted together in a yarn. For example, 2 ply yarn is formed of two strands, while 3 ply yarn is made of three strands. One can easily determine the ply of a yarn by untwisting the end a bit. Most denim is woven from 2 ply or 3 ply yarn.
Here is an illustration of ply counts: