Entropion is a deformity of the eyelid in which the skin folds inward and brings the eyelashes into contact with the cornea, causing eye irritation and potential vision damage. (It also occurs in humans, cats, and dogs - it's particularly common in short nosed dog breeds).
In horses it is seen most often in newborn foals as a congenital defect (but it sometimes develops in adults that have had a chronic eye problem). It's a hereditary problem, although generally not considered worth culling otherwise good lines. The most obvious symptom is excessive tearing. Usually, it's treated by suturing between the skin and the eyelid to pull the eyelid into a normal position, although sometimes more complex surgery is required. An antibiotic eye ointment might also be needed.