When referring to "ply," we almost immediately think of toilet paper. Not to say we shouldn't, as stacking wood or paper denotes a certain amount of plies that we associate with lumber and tissue.
However, a piece of yarn can also be called a ply. That yarn is then loomed into fabric as we know it. A single ply fabric made of resilient and high quality wool is a wonderful textile. But of course that isn't the only way to mill fabric.
A 2-ply (or 3 or 4 etc.) is made by twisting two single ply yarns together to create one yarn. This has a number of benefits, but the biggest advantage is that the yarns will perpetually want to untwist, which reduces and often eliminates wrinkling. And because of the way the fabric is milled, the yarns will never actually untwist. Another advantage is that while the fabric is heftier, the 2-ply construction is a more open weave. This allows air to pass through the weave and creates a more breathable textile.
Pictured here is a 2-ply, or Fresco, fabric from Ariston. We used an Olloclip magnifier at 21x to view the individual warps and wefts. You can see multiple colors twisted into each yarn (cream/tan, grey/black, brown/caramel, royal/dusty blue) which only accents the depth of this particular fabric's color and pattern.
Next time you're in the store, please ask to see the fabric under the magnifier to get a better understanding of how your suit is made.