A public key is like an open box with an unbreakable lock. 
If someone wants to send you a message, they can place that message 
in your public box, and close the lid to lock it. The message can 
now be sent, to be delivered by an untrusted party without needing 
to worry about the contents being exposed. Once I receive the box, 
I'll unlock it with my private key - the only existing key which can 
open that box.