It's a nonsense phrase used when someone is upset or sad. You pat them on the back and say "There, there." You can also say "there, there, everything will turn out right."
It's a rather rude way of saying "be quiet" - it's referring to tossing something into a trash can, as in "what you're saying is garbage so shut up."
in on it means you know whats going on or about a certain subject."when it comes to penguins I am in on it!" i might say
I think it means that that person agrees with that others persons idiom and that it fit that question that the teacher or whoever asked that question.
It's kind of like "blew his top", which means that he/someone had an outburst of anger.