What does Weight mean?
The grade of denim in terms of its weight per yard of fabric.
Heddels explains Weight
The weight of denim is determined by how much one yard of fabric weighs, and ranges from light to medium to heavy. Most denim weighs between 5oz. and 20oz., with 5oz. being a lighter denim and 20oz. a heavier denim.
Light denim is usually used for shirts, skirts, dresses, and other garments that need to drape well while thick denim is used more for heavy trousers and non-clothing uses. Jeans are typically made of 12oz. or 14oz. denim, though there are many brands that use a higher or lower weight of denim.
Pile of denim of various weights: