When you send Steem or SBD to someone, you can put whatever you want in the memo field. It's basically just a line of text for sending a note along with the money. For premium upgrades, the memo is optional, we don't really need it as we know what your payment is for. But lots of people choose to put a memo anyway for clarity.

You should never put any kind of key in the memo field. Luckily, you put a public key not the private one, so no harm done. In order for someone to gain access to your account they would need the private key.