Parties exchange wallet information to complete transactions. (due.com)
If you want to store a large amount of money securely you can do so on paper by printing out a couple QR code that contains the entire wallet information. (arincrumley.wordpress.com)
It's basically a small hard drive (which would be indestructible, in the best case scenario) that securely contains your cryptocurrency wallet information. (myintimcity.com)