It's a little known Steemit feature few people are aware of. Short version: Let's say you need to send someone your address here, and you don't want the whole world to see it.
So, we can include a memo when we transfer funds with our wallets. If you start the memo with "# " (hashtag + space) the content of the memo is encrypted so only the sender and recipient can see it. Everyone else-- logged in or otherwise-- will just see a blank memo field.
If you use a blockchain viewer, I believe the memo field will just show up as a long string of jumbled characters. Now where the encryption key actually resides, that I don't know.
Wow! Yes, I definitely didn't know that. Thanks for the information!
Thanks so much for asking the question @customnature. I wasn't aware of how to keep a memo private either. Something you would obviously need to do when supplying details like your address or phone number.