>>2293283what
>>2293283 said, but for fly fishing specifically you want a decent amount of your spool to be backing because fly lines are usually only around 90ft long and if you hook into something that wants to run, you'll have another 50+ yards of line to play with. Backing also has a second purpose of making your arbor diameter larger, meaning the fly line is stored in larger coils and you'll take home more line per turn.
Pic related, the bright green line on the reel is the backing.
