Fill up your reel with the Airflo Micro Poly Backing. AMPB is the most popular backing material on the market, coated to ensure it's longevity.
Fly line backing serves two basic purposes. The first is to fill up your reel so that your fly line completes the filling of your reel to within, say, 1/8th inch of the spool capacity. The second is to still stay connected when a big fish strips of all your fly line!
For most purposes, the No Frills backing is perfectly adequate as a trout backing line. If you require a stronger line, say for salmon fishing,then you should consider another backing line on this counter. If you are using small reels and have limited space for the backing but require a long length of it for big fish - then, again, you require one of the more advanced backings.