This is a New Chums Chamois Lens Cloth.

No more wiping your sunglasses or glasses on your t-shirt or trousers and scratching the lens. It can easily attach to any loop on your clothing to be used to clean your goggles, sunglasses or mask. Chamois will not scratch either mask or glasses. It is approximately 6" x 6" and tucks back into its case when not in use.

This is simply a fantastic product that comes in handy all of the time!