Anyone who wants to add to or correct this feel free...
Generally, a tape reel consists of a hub (the round part in the middle around which the tape is wrapped) and two side walls called "flanges" that hold the tape "pack" in place once it's wrapped around the hub. Very often if you look at an empty reel you will see a small slot in the central hub part. The original purpose of the slot was to hold the first little bit of tape so that once winding began, the layers of tape wouldn't slip on the rotating hub. However, tapes that have been wound on "slotted hubs" and stored for any length of time often show a problem in the form of slight tape deformation near the hub. The little indentation of the slot itself can make a crease or dimple on the first layers of tape, and these can in turn cause several inches of the tape pack to mirror the same crease. In other words, the nice smooth wind that assures even tape-to-head contact is lost, and the crease can be heard in the form of a dropout as the tape passes over the playback head. Audio archivists have long recommended the use of slotless reels-- that is, reels with slotless hubs. The tape is held in place not by tucking the end into a slot but just by surface tension as it wraps around the hub. Using slotless hubs results in a much smoother wind generally and does away with the crease that can permanently deform the tape.
Hope this helps!