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.