What does "overhaul" mean? Does it mean to reorganize something? Could you give me some examples?

What does "flat" mean? Is it more of a British English?

Yes it is! A "flat" is a British English word that means apartment. A "flatmate" is someone that lives with you; roommate in American English. Ex: They live in a two bedroom flat. Ex: Her flat is just down the block.

What does "mess up" mean? and what else can I use instead of "mess up"?

"Mess up" means "make mistakes" or "ruin", depending on the context. You can say "make mistakes" instead of "mess up" when referring to a person's actions, and use "ruin" when something physical is being distorted. Ex: I always make mistakes when taking tests! Ex: I ruined my shoes when I stepped in mud. Ex: The wind messed up my hair. Ex: It won't be too long until the new guy messes up.

What does "overhaul" mean? Does it mean to reorganize something? Could you give me some examples?

"Overhaul" means to redo, start again, or repair something. In this case, it means to redo and create a completely new menu. Ex: They overhauled the restaurant interior! It looks so amazing now. Ex: I sent the ice cream machine in to be overhauled. Ex: They did a complete overhaul with the operating system. I have to learn how to use it again!

What's "unagi" referring this sentence? Doesn't it mean "eel"?

Unagi is the Japanese word for "eel", but Ross is calling "a state of total awareness" as Unagi. Ross studied Karate for years, and he thinks self-defense is useless without Unagi.

What's the main difference between "bag" and "sack"? Can't I say "sack" in this context?

In this context, you could use "sack" instead of "bag". A "sack" is a type of "bag". You use "sacks" to store food in large quantities, like the corn in this case. It is a large bag made out of strong material, like canvas. Ex: There was a sack of flour in the cupboard that I couldn't reach. Ex: Can you get me a small bag of crisps for the road? Ex: Sarah thought she could turn the old sack she found into a doormat.