Healthy meibomian glands are absolutely vital to a healthy tear film - and misbehaving meibomian glands are probably responsible for the majority of dry eye symptoms. If you have chronic or uncontrolled dry eye symptoms and have never been told about meibomian glands, don't be surprised but do take the opportunity to educate yourself, right now, about meibomian glands and meibomian gland dysfunction.
Please see our Dry Eye for Dummies article for a general introduction to the layers of the tear film and the role of the MGs. But briefly... in simple terms, the tear film is comprised of three layers: 1) a watery layer sandwiched between 2) an oil layer on top, which is produced by the MGs and which keeps the watery part from evaporating too quickly and 3) a mucin layer underneath, which "glues" the watery layer to the surface of the eye.
When meibomian glands aren't producing their full quota of oil (due for example to inflammation or atrophy) dry eye symptoms ensue. Promoting good meibomian gland health is often the number one task of the chronic dry eye patient. See meibomian gland dysfunction for more information.