CBD flower is the original source of cannabidiol. Hemp plants are typically rich in CBD, in addition to other cannabinoids and terpenes. They also contain some THC, but not enough to actually get you high, and typically within the legal limitations of 0.3% THC.
CBD flower refers to the buds of hemp plants. Hemp generally contains around 15-20% CBD, in addition to a full spectrum of other cannabinoids. It is virtually indistinguishable from marijuana—the main difference is that hemp has lower amounts of THC. The 2014 Farm Bill has legalized the cultivation of hemp in the USA, and it is now sold in every state—although some states may require a prescription to obtain CBD-based products.