When fly fishing, backing is the line that is stored on the reel behind the fly line. It is used to provide extra line capacity in case the fish makes a long run. The best weight for backing depends on the size of the reel, the weight of the fly line, and the type of fish you are targeting.
Generally, the backing should be about the same weight as the fly line. For example, if you are using a 5-weight fly line, you should use 5-weight backing. However, if you are fishing for a large fish, such as a tuna or a marlin, you may want to use a heavier backing. This will help to prevent the fish from breaking your line.