It is a cheaper line that you put on the reel first. Whether it’s saltwater or freshwater fly fishing, you’ll rarely cast far, so your main fly line can be quite short, usually too short to fill the reel properly. You don’t want to be buying loads of the expensive stuff, so backing does the job.; You then tie your main fly line to the backing. It’s more cost-effective, and in the event you do hook a big one, there’s little danger you’ll run out.