You have defective eyesight in one way or another. The value of the trait determines how bad the defect is.
-1: A minor fault. You are for instance slightly myopic and can't discern details more than up to ten steps away.
-2: You are gravely myopic and can't recognise individuals if they are more than an arms length away, or you have lost your eyesight entirely on one eye.
-3: A grave defect that means you only have leading vision, which means you can only determine between light and dark and see silhouettes, but you cannot identify anyone even at close distance.
-4: You are completely blind.