What does "Have in common" mean?

Is 'sit back' same as 'sit down'?

No, they're not the same! Here "sit back" instructs the dog where to sit. So rather than sitting in the front of the car, to sit at the "back "of the car. "Sit back" can also be used to tell someone or people to relax while already sitting - to become more comfortable in their chairs. "Sit down" is more of the action of sitting from a standing or other position. Ex: The teacher told us to sit down and take our seats. Ex: I sat forward during the whole movie because it was so good. Ex: Sit back and enjoy the ride.

What's "it" in "that's it" mean here?

"That's it" is an expression, so the "it" doesn't really mean anything on its own. The expression has the same meaning as "Is that all/everything?" to ask if the whole situation or event was described and if there are any other important details? "That's it?" implies that the speaker hopes there isn't anything more. It's usually followed by a statement that is used a question, to confirm is "that" (everything that has happened or been described) is "it" (the information is complete and there is no additional information). "Is that it?" is more neutral, and the speaker is asking if there is more with less emotions attached. Ex: What you told me about the conference call, is that it, or is there more? Ex: So, that's it? We're done with the project?

Does “rocks” refer to subject "geology"?

In this scene, the speaker is making a joke about getting a "C- in rocks". Part of the joke is that a class only about rocks and not even the science behind them doesn't exist, and she didn't take a class at all. Therefore, "rocks" is simply referring to rocks in this scene, not geology as a whole.

Which one is right, "go to a party" or "go party"?

The word "party" in the phrase "go to a party" is a noun. So "go to a party" means to go to a physical place where there is a party. In the phrase "go party", "party" is a verb, so this phrase means to go and act in a manner of partying, which is usually crazy and wild. In this case, she is talking about attending a physical party, so she uses the phrase "go to a party". However, both phrases are correct depending on the context and what the speaker is trying to express. Ex: Let's go party tonight! Ex: I am going to a party in the city tonight.

What's "humbled" mean and when's it used?

"Humbled" means to be lower in dignity or importance due to something or someone; it makes one understand they're not as important as they think. Here, it's used in the sense that this event is so big, that it reminds her that she's incredibly privileged to have the moment. Ex: Hannah was immediately humbled when she met her celebrity idol. Ex: He humbled himself and asked for help.