Hi mriaz1:
Structure refers to how the author constructs his/her passage to make an argument or to discuss an idea. Identifying structure takes practice, but being able to do so will help you a lot in understanding RC passages.
Let's give you an example of how to identify structure:
First sentence of paragraph 1: "Most people think that ice cream is bad for your health..."
First sentence of paragraph 2: "
However, studies show that ice cream can serve as an excellent calcium supplement to a healthy diet..."
Here's how I see the structure of this essay: the first paragraph talks about a commonly held belief, while the second paragraph changes the direction of the passage to counter the first paragraph. Notice the word
however. This word has the structural purpose to change the direction of the argument for the reader.
I hope this example helps clear up your confusion. Overall, identifying structure is all about recognizing how an author designs his/her essay to talk about ideas. The
Princeton Review Cracking the GMAT book has a decent overview of structure in its Reading Comprehension section. It may be a good reference for you.
Best of luck!