Yes, exactly. It's an example of something which is very difficult for most people to understand. It is used emphasise the fact that something which you are talking about is very easy. For example, if you are getting frustrated that your friend can't understand something which is very simple, you might say 'Come on! It's not rocket science! Why can't you do this?'

A message commonly intended to show humor, is this sort of thing: "You do not have to have a Doctoral Degree in Rocket Surgery to understand the problem."

aerospace engineering, or Rocketry. That is what the technical meaning is. However it is also used informally to refer to something very difficult (example: "this algebra class is like rocket science to me")