A chalazion is a painless lump resulting from the blockage of an oil gland in the eyelid, often caused by inflammation and thickening of the gland's secretion. Common in adults aged 30-50, symptoms include swelling, tearing, and potential blurred vision, with various treatment options from warm compresses to surgical intervention. Understanding preventive measures, such as daily cleansing and protecting the eyes, is crucial to reduce recurrence.