A startle reflex is where the fish flinches away when the tank is bumped, the light comes on suddenly, something moves toward the fish suddenly, etc.. This is the normal behavior of fish.
If this reflex is missing or the fish doesn't seek cover when it appears threatened, and the other fish of it's species do, than it could incicate a blood flagellate disorder (sort of a fish version of sleeping sickness), the recommended treatment is 5 drops of methylene blue per gallon of water for five days.