CBD stands for Cannabadiol and is one of over a hundred phytocannabanoids found in the flower of the Cannabis plant. Without going into a full biology/botany lesson, it should be noted that phytocannabanoids are simply naturally occurring compounds found in the cannabis plant. CBD and THC (Tetrahydrocannabinol) are the two most noted of these compounds and the two most studied.
So what is CBD, and why is CBD so popular lately? There are certain aspects that set CBD apart from other compounds found in marijuana. As medicinal marijuana research gains in popularity, CBD is one of the main compounds being studied. There is already one CBD drug Epidiolex, used for childhood epilepsy, approved by the FDA. https://www.nbcnews.com/health/health-news/fda-approves-cannabis-based-drug-epilepsy-n886386
Another aspect to CBD that makes it especially attractive to medicinal researchers is, it does not produce any psychoactive results. This means that CBD in pure form will not make a person feel intoxicated. THC is the compound responsible for that.