Following The Investment Order - Couple Of Questions

[NeoGenMike]

Hey everyone! I've been doing the best I could until I discovered MMM.com and a whole new array of things have opened up to me. I've, just now, taken a look into following the investment order that has been stickied to this subforum and I had one or two questions if anyone would care enough to donate the time.
What would you think is a good emergency fund? Let's say someone spends 1500 a month. I was going to keep 15k or so. I've never lived on my own, so I'm not 100%.

Now the real questions. Could someone explain, in a layman's term what an HSA is and (at 27) do I need one? And the same goes for a Mega Backdoor Roth. I think I get it. It's just a way to sneak in extra money from your tax-free 401K into your taxless withdrawal Roth. Buuut I would like this explained to me like I had no idea what any of this is. I like how you guys explain things, haha.

Thanks for your help, Everyone. I've learned so much since I came in.

[MDM]

What would you think is a good emergency fund? Let's say someone spends 1500 a month. I was going to keep 15k or so. I've never lived on my own, so I'm not 100%.

$15K seems plenty, but see Emergency fund - Bogleheads.

Now the real questions. Could someone explain, in a layman's term what an HSA is and (at 27) do I need one?

See Health savings account - Bogleheads.

And the same goes for a Mega Backdoor Roth. I think I get it. It's just a way to sneak in extra money from your tax-free 401K into your taxless withdrawal Roth. Buuut I would like this explained to me like I had no idea what any of this is. I like how you guys explain things, haha.

See The Mega Backdoor Roth IRA - Bogleheads.org

You may detect a trend in the answers above.... ;)

[NeoGenMike]

Ah yes! Thank you, haha.

[ChpBstrd]

Your emergency fund should be large enough to cover emergency-level spending for the few days it takes money to transfer from your brokerage to your checking account. In theory, this could be one day if you did a wire transfer.

Emergency spending might include a hotel, car rental, clothing and hygenie purchases, a prepaid phone, various fees, a few days of restaurant food, and a few hundred dollars for medical copays.

Some people say why have ANY idle cash? Get a credit card for emergency situations and pay if off in a month from brokerage proceeds. This is a big debate.