Cube keychains are accessories that one can attach to zips, rings, or anything with a hook. They usually consist of a ring to attach to other things, and an object attached to the ring which has some sort of significance. Usually, people get keychains that symbolize something that means a lot to them, and they enjoy carrying it with them everywhere they go.
In the cubing world, keychains usually have objects like twisty puzzles of various types, wristbands and other toys as well. The fun part about having twisty puzzles as keychains is that you can solve them anywhere, and can always carry them with you at all times.
The most famous keychain-cube is the QiYi 3x3 keychain, which has a mini-3x3 attached to it, and is extremely fun to solve. The keychains also have a very aesthetic look to them, with the puzzles attached to them also having a fun element in solving them.