The reel does three things: holds your line, lets line out, and applies friction (drag) when a fish runs.
Fly reels are usually click-and-pawl or disc-drag, with line hand-stripped during casting (the reel mostly stores line). Spin reels are open-faced or closed-face, used to crank in a lure. Baitcasters mount on top of the rod, with the angler thumbing the spool to control casts. Match the reel size to rod weight or species — too small and it lacks line capacity for big runs.