Blepharitis is a chronic inflammation of the eyelids, often leading to meibomian gland dysfunction and dry eye symptoms.
Symptoms of blepharitis may include red, swollen eyelids; dry eyes or eye pain; and crusting around the eyelids/lashes on waking.
Blepharitis is a very common condition and can occur in such mild forms that you don't notice it. The best way to treat blepharitis and to prevent it from recurring is to do lid scrubs and, if necessary, warm compresses.
Blepharitis causes dry eye symptoms by interfering with the secretions of the meibomian glands, which produce the lipid (oily) layer of the tear film that prevents tears from evaporating too rapidly. That is why the stinging or foreign body sensation associated with dry eye can be felt even when tear production (aqueous tears that is) is normal.