A subproduct is something made or obtained during the production of something else.
Subproducts are usually less important than the main product.
Examples:
The cheese factory makes whey as a subproduct.
The subproduct of making bread is the leftover dough.
The subproduct of making juice is the pulp.
The subproduct of making a cake is the eggshells.
The subproduct of making a car is the scrap metal.
The subproduct of making clothes is the fabric scraps.