Ply stands for plied yarn. The number represents the amount of strings twisted around eachother. 3 PLY means three threads, resulting in the followjng cut:
A high number of plys does not per se mean that the thread is thicker. The individual threads can still be thin or thick. The more threads there are in a yarn, the firmer it becomes. The thread itself becomes less airy and the thread becomes heavier. There will be fewer meters on a sphere.
Yarn count refers to the weight of the yarn. It can be expressed in many different formats which mostly just relate to different metric systems (meters, feet, kgs, ounces etc.). Which format is used depends on the purpose and material. Sewing thread is often expressed in dtex, rug yarn in tex, nM or Dewsbury.
But overall every format does the same, it explains the weight per length unit. A yarn converter can help you translate it to a format that makes sense to you 😄
You can find a helpfull tool right here: