Eyecups are simply shades that prevent extraneous light from distracting your vision or entering the eyepiece while using your binoculars. Most consumer binoculars have soft rubber eyecups that can be folded out of the way or simply removed if you don’t want to use them or if you wear eyeglasses. However, higher-quality binoculars often have plastic or metal eyecups; these can accommodate eyeglasses and are adjustable to the most comfortable position for personal viewing.
If you have lost an eyecup on your current-model binoculars, contact Celestron tech support, as they may be able to ship you a replacement. If you need eyecups for older models, check a local camera store's junk bins for one that fits.