Made of three layers stuck together, or three strands twisted together. A 'ply' is one layer or one strand, so three-ply means there are three of them.

You meet it in a few everyday places. Toilet paper and tissues are sold as one-ply, two-ply or three-ply, and the higher number means thicker and softer — this is the use you see most often on packaging in a supermarket. Wood: three-ply plywood is three thin sheets of wood glued together, each with its grain running a different way, which makes the board much harder to bend or split than one thick sheet. Wool and rope: three-ply yarn is three thin strands twisted into one thicker strand, and knitters care about it because the ply number tells them how thick the finished knitting will be.

It is an adjective and it goes in front of the noun: three-ply tissue, three-ply wool. You would not normally say 'this tissue is three-ply' in conversation, though it is not wrong — the phrase lives mostly on labels and in shops. The whole family works the same way: two-ply, four-ply, six-ply. Keep the hyphen.