Filamentary keratitis refers to a condition where strands of mucus stick to the surface of the cornea. This causes eye irritation and a foreign body sensation, especially when blinking.
Any of a number of conditions which affect the surface of the eye can cause this to happen.
Such conditions include aqueous tear deficiency, corneal exposure (inability to close the eye fully), previous eye surgery such as a corneal transplant, and miscellaneous other conditions such as severe allergic conditions associated with bumps under the eyelid called papillae.
Do consult with a doctor for a comprehensive eye check up if you have this condition, as appropriate treatment can help a lot in improving the comfort of the eye.