If you understand what the OP is asking, and the question is reasonably answerable, then answer it.
This generally works out better if you are an expert in the subject matter. Some questions are clear to experts in the language or technology, but unclear to those who might otherwise be able to answer the question had it been written more clearly.
For questions that are evidently unclear (expert knowledge notwithstanding), you should ask for clarification and refrain from answering. Cast a close vote if it is apparent that the question is not answerable without clarification (i.e. don't guess).