Almost all encryption protocols have some concept of "keys". These keys are what allow you to encrypt and decrypt data. Like any other kind of key, it is imperitive that you keep these keys safe and do not lose them. If you do, anyone who obtains them will be able to encrypt and decrypt data as if they were you.
The key based encryption method we will be talking about in this chaper is what is called asymmetric cryptography. In asymmetric cryptography you have two keys. The first is a publickey which should be available to the world. The second is a privatekey which you (and only you) must have access to.