The terms 2-ply, 4-ply, and 6-ply refer to the number of layers, or "plies," of rubber and fabric used in the tire's construction. The greater the number of plies, the sturdier the tire.
For instance, 2-ply tires have two layers, making them lighter but less robust.
Meanwhile, 6-ply tires have six layers, which makes them more robust. As a result, higher-ply tires can carry higher loads and have better resistance against punctures, but they are also heavier, leading to worse performance and higher fuel consumption.
Today, you can also find 8-ply, 10-ply, and even 12-ply tires for some really large and heavy-duty trailers.