The term pile refers to the thickness or density of the fibers in a rug or carpet measured from the backing to the surface.
Pile heights vary from low, flat/short piles (less than ¼ inch), to medium/thick piles ( ¼ inch to ½ inch) to thicker/plush piles also known as shags ( ½ inch to ¾ inch)
Note that a pile thickness does not determine its quality.
There is however more strength in a shorter pile rug, thus giving it a longer life.
Cleanability of different piles vary but the shorter the pile the easier it is to clean.
The longer the pile the higher a vacuum rotating brush setting should be and if possible turned off. Longer piles are also susceptible to crushing, shading and furniture marking.