Arbor refers to the cylinder at the center of the reel that the backing and fly line is wound around. Most modern reels feature some type of large arbor as they retrieve more line faster and create less ‘memory’ or twist in the line and leader. If you are chasing warm water or saltwater species, pick a reel with a larger arbor as this will aid in smooth runs as well as retrieving a lot of line quickly when the fish turns and runs back at you.
Classic trout reels are generally smaller in arbor, as are beginner fly reels. They are usually lighter, less bulky, and less expensive than their large-arbor counterparts.