What is yarn pilling?
Pilling is the formation of fuzzy balls on the surface of knitwear, which is caused by how the yarn is made. To create yarn, numerous fibers are spun in a long thread.
Wearing and washing a garment can cause loose fibers to push out from the surface and form into a small ball or pill.
Friction is the main factor in causing yarn pilling. 
Therefore you will often see pilling under the arm of a sweater or behind the heel of socks.
While certain types of yarns or stitch choice can result in less pilling or delayed pilling, this will happen to just about every yarn (natural or synthetic)!
Fortunately, there is an easy way to extend the life of all your favorite knitwear by using tools as the Gleener Fabric Shaver & Lint Brush.
The Gleener requires no batteries or electricity and will help to get rid of pilling, lint and pet hair from all fabrics (not only your knitwear!).