Not always.But it is a very common everyday expression. I don't get what you mean.I don't get you. (I don't understand what you mean,I don't understand you.) Teacher: Do you get it? Yes we do.

I second that. To use the verb "to get" in general (ie to mean to obtain/achieve/have or to understand) is very informal.

Something worth bearing in mind is the fact that 'get' is very informal language. Using 'get' to mean 'understand' in fine in casual and colloquial conversation, but you should avoid using it in polite or formal contexts. So in this respect, the answer is no - not always.