A coupon is an informal term for a small sample of material, most often metal. It’s perhaps most commonly used in welding where practice welds for people first training to weld are done on pieces of metal often referred to as coupons. :)

It is usually an artifact of materials like metal which are hard to cut, and can’t efficiently just be cut as needed during a task, but are prepared and pre-cut into small pieces you can grab easily. Each of those pieces is called a coupon.

I used to store mine in a Chinese takeout container box.