“Describe your personal and professional network.”
My question is: What would the interviewer be looking for in the candidate, in response to this?
We can only imagine what the other person is thinking ...
What would they expect you to say about people you associate with, something negative?
Unless these people are going to affect your work, or you'll be expected to recruit them, or tell them what a great company it is then I don't see how the question is either universally applicable or specifically useful when recruiting a junior Engineer - was your Engineering School known for stunts/pranks/problems?
Obviously you'll want to provide a short list of useful and interesting people whom you've not been in contact recently and provide a positive tidbit about each of them.
Sort of like: "Tell me about yourself ...", but it's "tell me about whom you know".
It also depends upon how the interview seems to be going ...
They might be trying to get you to engage in conversation and assess if you seem like a likeable person. I been to so many interviews where it seemed like the owner really didn't know much about the subject at all, they just wanted to chat to see if they liked you before handing you off to someone else.
It's tough to assess these things without more context, so you might consider replying with the generic answer I offered 4 paragraphs previous.