The meibomian glands, lying behind eyelash follicles, secrete meibum that helps prevent the protective eye tear film from evaporating. The secretion creates an oily edge along the eyelid keeping tears within the eye and making the eyes airtight when the eyes are closed.
When the meibomian glands don't function correctly, posterior blepharitis results, with symptoms of dry eyes. Clogged meibomian glands can result in inflammation and in turn, a cyst known as a chalazion.
Each time you blink meibum is pumped onto the surface of the eyes. This is the reason that the tendency to not blink while on the computer or intently looking at something contributes to dry eye syndrome.
See more about the internal functioning of the meibomian gland.