A question is posed and an answer is supplied. If the response does not actually answer the question, is this what is meant by materially non-responsive?
Materially non-responsive means that the answer was "non-responsive" (i.e. it didn't answer the question being asked) in a significant and relevant manner. "Material" means in a matter this is significant and relevant to the issue being considered in the relevant context.
For example, if someone is asked, what is the state and city where this incident occurred, the answer "somewhere", while technically true, is materially non-responsive as it doesn't provide enough information that is available to the person asked to be a meaningful answer to the question of which city and state was involved, while the answer "Denver" might technically be non-responsive since it doesn't include a state, but this is not "materially non-responsive" if "Denver, Colorado" can be inferred from context.