What does 'rah-rah' mean?

When can I use the expression "keep the change"?

The expression "keep the change" can be used whenever you pay someone with cash and you want them to keep the full amount that you paid them instead of them giving exact change back to you. This expression is used to indicate that you want to tip the person from the amount that you paid them, so they can keep the extra money they owe you as a tip. Ex: I gave my taxi driver a $20 bill for the $10 ride and I told him to keep the change. Ex: Here's 10 dollars to go pick up a coffee for me. Keep the change!

What does the word "imposter" mean?

An "impostor" is a person who pretends to be someone else in order to get or achieve something. An "impostor" usually seeks some kind of financial gain when he or she assumes another person's identity, but there can be other motivations, like simply the thrill of doing it. The original meaning of the work is to "impose upon or deceive." It's also sometimes spelled imposter, although both spellings are correct. Ex: He claimed he was an experienced pilot, but he turned out to be an impostor. Ex: The man who claimed to be a prince turned out to be an impostor.

What does "even" mean here?

In this video, "even" is used to emphasize her statements. In the similar context, "even" is commonly used to emphasize anger, severity, or surprise. Ex: Don't even think about it. Ex: He has never even heard of Jennifer Anniston! Ex: You don't even have a chance of winning the lottery.

Could you tell me some similar expressions to "carry on"?😗

To "carry on" means to continue doing an action. This expression is very common in British English. Some synonyms for "carry on" are "keep going", "continue" and "proceed". You can also put any verb after "keep" to express that an action should continue happening. Ex: Keep talking, I want to hear more of the story. Ex: Keep going, you're almost there!

Is no verb needed in the first part of this sentence? What's the grammar rule for that?

Yes, that's correct. The "what [a]" here is used to emphasize the noun/object that follows, so a verb is not needed here. In this video, "what a cool idea" is used to emphasize how cool of an idea it is. Ex: What luck! I just found a four-leaf clover. Ex: What a busy day! I'm so glad I can go home and rest now.