A small safe that contains your sim card or microsd card while travelling. It attaches to your keyring which locks it shut.
There are 4 sizes.
They fit together while the keyring hole is 90 degrees off. Then rotate it 90 degrees so the keyring holes align. Then attach it to your keyring to lock it shut.
When I travel I keep my home sim card in it to keep it safe while I use a local sim card.