CBD is a non-psychoactive substance taken from the Cannabis Sativa (or Hemp) plant. CBD stands for Cannabidiol, one of over a hundred cannabinoids found in the plant.
There are estimated to be over a hundred cannabinoids in the plant. CBD tends to be from strains high in CBD, but low in THC, the active ingredient in marijuana. It is then further refined so as to contain less than 0.2% of THC, so that it is completely legal in the UK.