Nolan, I assume you mean this large cylinder? I don't know if you are kidding or not about not knowing what it is...if you are serious, it is an oil filter. It is only slightly more useful than no filter at all as it only filters oil that bypasses the main gallery flow, or about 8-10% of the flow.....hence, you see them removed and external filters fit on most performance motors. And by the way, that is a generator, not an alternator while I am being petty.......most of us who ever had one of these wishes it was an alternator......the generator provides much less current than an alternator....
and if you are not serious, it is still an oil filter (:>)