Huge mistake IMHO. But I'm sure @Freddie could elaborate
"It means its a single layer. Its very thick and soft and cleaning shouldn't be a problem. In fact some of the popular 2-ply ones start to disintegrate as soon as they get wet, so this is my preference."
Or another response I got was,
"there is only 1 ply (1 layer) of tissue. some brands are 2 ply 3 ply etc, meaning multiple layers of thin tissue pressed toghter to create the sheet of tissue. this brand is 1 but it is very thick."