Rod and reel compatibility are key when choosing your reel. The reel needs to hold the necessary amount of backing and fly line and match the weight of the rod you're fishing with. Choosing the correct size reel will also ensure that it's weighted properly to provide the proper balance with your rod.
A reel's drag system applies resistance and regulates the speed of the reel when a fish runs and starts pulling on the line. Most reels use a disc drag system, which slows the spool by applying pressure and creating friction between two discs. The bigger the fish you're after, the more critical it is that your reel have a solid drag system.
The arbor is the spool on which the line is wound. Traditionally, all reels were a standard size, but large arbor reels are becoming increasingly popular. A large arbor allows you to retrieve line faster and more easily than a smaller arbor, and it helps reduce memory in the line so you get fewer tangles and more consistent drag.