Let's start with the less scary of these two words: keys. In the whole Planet Nine system you interact with software via a _private_ key that you hold onto (and keep safe), and a _public_ key that you can give out to the software that you want to use.
It is not incorrect to think of the private keys like the keys on your keychain. In this metaphor, your public keys are locks that you install in programs and on servers. I like thinking about it this way because it highlights one of the crucial distinctions between many cryptographic key implementations, and the Planet Nine system. Namely:
YOU ARE NOT A KEY.
You are a human, and likely a complex and interesting one, which I will never get to know fully. Planet Nine will not get to know you either, because it collects no information about you. The keys you create have as much to do with you as the keys you hold in real life. They probably represent a loose association between you and something within your sphere of concern and nothing more.
Put another way, keys are not identifiers. They're just keys.
And once we're feeling comfortable with that, let's talk Cyptography.