Is asking your PhD supervisor a basic question considered inappropriate?
If your colleagues in the department—presumably also PhD students—don't understand the concept, either, it doesn't strike me that the question is truly "basic." That said, if your advisor is aware of your background, then he should know that there will be some things that might not be "obvious" to you. 

Now, in this case, you have already "done your homework": you've searched for an answer on your own but couldn't find one. Mention this to your advisor when you ask the question. A good advisor will recognize that you have done what you're supposed to and provide some guidance to help you. He may not provide you all the answers but should at least be able to point you in the right direction.