A fly line backing is basically a regular fishing line that sits between your fly line and the reel and acts as a reserve line, just in case a fish is strong enough to pull all of your regular line off.
Firstly, you want to choose an appropriate weight. Most backings come either in 20 lbs or 30 lbs. 30lb backing is great for heavy-duty applications, such as fishing Alaska salmon, or it can be used for lighter applications when you don’t need too much line to fill your spool.
Also, think about how much backing you want. Obviously, you need to consider the capacity of your fly reel first. But it should also be selected according to the size of fish that you're planning to catch. If you're targeting smaller fish, 75 to 100 yards of backing should suffice. However, if you're planning to reel in larger fish, like salmon or tuna, you should consider using a fly reel backing of at least 200 yards.
Next, consider the material of the backing. Most backings are made of either Dacron or braided Spectra (aka Gel Spun). Dacron is soft, supple, and slides very nicely, while Spectra is thinner and stronger but harder on your hands.
At GRITR Outdoors, you’ll find backing from Scientific Anglers, Cheeky Fishing, and other trusted brands.