Source:allaboutvision
A chalazion is a benign, painless bump or nodule inside the upper or lower eyelid.
Chalazia (plural for chalazion) result from healed internal styes that no longer are infectious. These cyst-like nodules form around an oil gland (meibomian) within the eyelid.
The contents of a chalazion include pus and blocked fatty secretions (lipids) that normally help lubricate the eye but can no longer drain out.
Many chalazia drain, resolving on their own, especially if you facilitate the process with periodic warm compresses and gentle massage of the eyelid.
However, some chalazia persist for more than several weeks and grow large enough to become cosmetically unappealing. A larger chalazion may press on the cornea, temporarily creating irregularity on the eye surface and inducing astigmatism. This can cause blurry vision.