http://torzcd47rw4qh36g4yqxvv2tmifgmu6jjalkyqz4e4lzzwtfdfc7qaqd.onion/rsa-crypto-cracked-or-perhaps-not-mobile-hacker-for-hire/index.html
So, you designate one of them as your “public key”, which you share with the world, and you keep the other as your “private key”. This means that anyone who wants to send you a private message can lock it up with your public key, but (assuming that you really do treat your private key as private), only you can unlock it.