In trichiasis, one or more lashes are misdirected toward the eye and rub against the ocular surface.  Each blink causes irritation, and in some cases, damage to the sensitive outer surface of the eye.
It is usually necessary to painlessly remove offending eyelashes under a microscope using forceps.  New eyelashes grow every 40-50 days, so this problem is often recurrent.  Persistent problems may require electrolysis or surgical interventions, although these options do not assure the condition will resolve.