Good question! The first number is the line weight of the rod. The second and third numbers are the length of the rod in feet and inches. So for instance, a 590 is a #5 fly rod and it is 9 feet 0 inches in length.
In terms of the line weight and what that corresponds to:
- A #5 rod is an all around trout rod.
- A #6 rod is a slightly heavier trout rod that casts trout streamers more easily. It is also ok for small salmon such as pinks.
- A #8 rod is the all-around salmon rod that can handle silvers, pinks, steelhead, pike and other large fish. You can trout fish with it but the fight won't be as exciting when you get a small fish on the line.
Hope that helps decode some of the fly fishing terms you see on a fly rod!