What does "get in" mean here? and how's it different from using just "get"?
"Get in" here is a phrasal verb which includes the verb "get" + the preposition "in". To "get in" can have multiple meanings depending on the context. "Get in" can mean to enter a place, to arrive at, or become involved in. In this case, "get in" means to get involved in something, so "get into trouble" means that someone did something that led to them getting involved in a punishment. Here's some examples of "get in": Ex: Get in the car, we need to go. Ex: Did you get in trouble for going home late? Ex: The plane gets in at ten o'clock.