Is "chirping" here a gerund? I don't understand the grammar of it.

When's "weird" used?

If something is "weird," it means that it's strange and unusual! So if you think something is strange or unusual, you can call it "weird." Depending on the context, it could have a negative or positive tone. Here, it's said more in a negative and cautious way since they're unsure if the dentistry is legitimate. Ex: I find it weird that they didn't acknowledge your efforts. Ex: I think I'm a little bit weird but in a good way. Ex: This food is weird. I don't like it.

What's the main difference between "jealous" and "envy"?

The main difference is that "envy" is when someone else has what you want. "Jealousy" occurs when something you already have is threatened and you don't want to lose it. It's common, though, to use "jealousy" for both of these things. Ex: I'm envious that you get to go on vacation while I work. Ex: Your girlfriend Sarah gets jealous when you talk to other girls, Andrew. Ex: Oh man! You get the new gaming system?! I'm so jealous. => common use of "jealousy" as "envy".

Is "hungies" a general expression?

No, "hungies" is not a general expression. This character says "hungies" to say "hungry" in a cute way. This term is only used on this show, I do not recommend using it unless you are trying to be funny.

What does "watch out" mean?

"Watch out" is a warning to be careful. To look around and avoid any dangers/hazards. An example sentence would be: Watch out for that snake! It's venomous!

Is the word "stray" used commonly? Is it same with "lost"?

Yes, they have similar meanings. Synonyms for "stray" include "wandering" or "random." It usually refers to people (or animals like cats and dogs) who don't have a fixed home and wander around on their own, like the "adventurers" mentioned in this video. Ex: The stray dog was very thin and dirty. Ex: I helped a stray tourist who had gotten lost.