Meibomion glands are oil glands, about 30 each eyelid, which open onto the eyelid margin behind the lashes. If a gland blocks it continues to produce oil and fills to form a swelling in the eyelid, a chalazion cyst.
Eyelid inflammation may develop suddenly as a reaction to oil leak into surrounding eyelid tissue or infection.
Small, symptom free chalazions may disappear spontaneously, while large inflammations may be painful and cause blurred vision. Chalazions are common, more so if blepharitis is present, and may recur.