Real currencies are backed by the goods and services people offer for a predetermined quantity of that currency. In a gold-backed currency, one of these goods happens to be gold, but the same also applies to fiat. The Chipotle in my area is "backing" the dollar by allowing you to exchange $10 for a chicken burrito. When sign an offer letter to your employer, you are also essentially "backing" the US dollar with your labor. Meanwhile, when goods or services exchanged with bitcoin, they still priced in fiat currency, so you're still backing your local fiat. The only thing Bitcoin is backed by is speculators who want to well Bitcoin to someone else in the future for more fiat.