The continuous production and drainage of tears is important to the eye's health. Tears keep the eye moist, help wounds heal, and protect against eye infection. In people with dry eye, the eye produces fewer tears or tears of a poor quality and is unable to keep its surface lubricated and comfortable.
The tear film consists of three layers-an outer, oily (lipid) layer that keeps tears from evaporating too quickly and helps tears remain on the eye; a middle (aqueous) layer that nourishes the cornea and conjunctiva; and a bottom (mucin) layer that helps to spread the aqueous layer across the eye to ensure that the eye remains wet. As we age, the eyes usually produce fewer tears. Also, in some cases, the lipid and mucin layers produced by the eye are of such poor quality that tears cannot remain in the eye long enough to keep the eye sufficiently lubricated.
This answer is based on source information from the National Eye Institute.