What is Memo in Steemit

Memos (in steemit specifically ) are the short messages attached with the crypto transaction (that is steemit tokens).

What Memo key is used for:

Memo key is used to decrypt private transfer memos or in simple words to create and read memos.

Where it fits in the security hierarchy:

Well it comes under master and owner key but separately, I mean separate diagram for it because it doesn't cover active key and posting key. This (Memo key) is exceptionally different (than active and posting keys) and specifically used for on Memos (short messages). It useless now and can be ignored like hardly few people use it. see here :
steem-keys-and-passwords-full-guide