Ragweed is a plant that causes a lot of people to have hay fever. If you are talking about marijuana called Rag Weed that is probably just what we call schwag or duff (or crap). It is just really low quality marijuana that was grown poorly, harvested too early or just the poorer parts of the plant. It is not a particular strain of weed. Any pot when grown poorly can be come schwag (bunk, commercial, whatever term describes low qual pot).