First: my apologies if I am starting way too simple and talking down to you.
The keys are what puts the "crypto" in "cryptocurrency." Math behind the scenes is what makes our accounts secure. Everything we do involves private and public keys. It's like the post office box for my library. Everyone has the address to send mail in to us, but only staff members have the key to get mail out. There are several levels of keys for different processes on our blockchain. Your master password mathematically generates the other keys. Keep it safe. Never use it unless you need to access wallet.hive.blog and change all your keys due to a security breach.
Your Owner Key also allows you to change lower level keys, so you should never need to use that master password anyway. You would probably use that instead if you ever accidentally post your other keys somewhere or get phished by a scam artist.
Your Active Key is most like our post office box keys. It is used for wallet operations, like sending, staking, or trading your tokens. Your public key is your user name we all see, in my case @jacobtothe and in your case @sydney2.
Your Posting Key is what signs your posts and comments. You usually use it to sign in to hive.blog, peakd.com, ecency.com, etc. and tells the chain it is you doing the posting.
Your Memo Key is useful if you want to send an encrypted memo with your transactions. Open a private/incognito tab to wallet.hive.blog and sign in with your memo key instead of your active key. Send a transaction with the '#' symbol at the start, e.g.
#This is a super-secret message no one else should read,
and send the transaction. It will look like an incomprehensible string of letters and numbers to anyone else. Since it is also a wallet transaction, you will need to confirm it with your Active key. If someone sends you an encrypted memo, you will also need to sign in with your memo key to decrypt and read it. I do not know whether changing your memo key makes old encrypted memos irretrievable or not. Maybe I should test that at some point.
Hopefully that helps.