Eyecups are simply shades that prevent extraneous light from distracting your vision or entering the eyepiece while using your spotting scope. Usually they are rubber and can be folded out of the way or removed if you don’t want to use them or wear eyeglasses.
If you lose an eyecup for a current model, contact Celestron factory service. They may have a replacement available. If you need eyecups for old models, try a local camera store’s spare caps and junk bin for one that fits.