What does it mean when a fish rolls on a fly?

When it is said that a fish rolls on a fly, what is meant is that a fish has risen from lower down in the water to investigate your fly, perhaps with the intention of eating it, or simply out of curiOSity. For some reason, the fish decides not to eat it, and either turns away from the fly (close to the surface) or it may even 'hit' the fly without taking it into it's mouth. Either way, it will cause disruption of the water around the fly (a 'boil'). This is termed 'rolling on a fly!'