Basically, the instructor is saying that too many essays just blather on and say a lot of things without having a clear, logical flow that connects the ideas. If you just start your answer by writing down the first idea that comes to mind and then add another idea and then another, you will wind up with a jumbled mess. You wouldn't build a house that way so don't build an argument that way either.
Instead of you just writing everything that comes into your mind, the instructor wants you to think about how to answer the question before starting to write. Try to develop a vision of how the different ideas will fit together to answer the question. So, for example, do you have one single argument with several supporting pieces of evidence? Do you have several arguments, perhaps one or two principal arguments and a few minor ones? Or do you plan to rebut the objections to your argument? These are different shapes that your answer can take.
Plan out what you are going to write before you write it so that the reader does not get confused and lost by a disorganized jumble of ideas that do not seem related to each other.