It is the result of overproduction of a particular line of fabric or sometimes small errors in the fabric printing that makes it unusable for its intended purpose.
Most quilt shops, either in the shop or on their online store will have sale items or a clearance section.
These can be either half-yard or fat quarter bundles usually made up of clearance fabric that needs to be moved on to make space for new stock.
Most quilt shops, either in shop or on their online store will have a clearance section for remnants, off cuts or bolt ends (sometimes called bale ends or roll ends too).