CBD is largely non-psychoactive, meaning it has no intoxicating effects. This has to do with how CBD interaction with cannabinoid receptors located throughout the body.
Like all cannabinoids and endocannabinoids, CBD works within our endocannabinoid system (ECS), which is a part of the nervous system. The endocannabinoid system is comprised of cannabinoids that the body naturally produces, called endocannabinoids, and two primary cannabinoid receptors.
The first receptor, CB1, can primarily be found throughout the central nervous system (CNS). The other primary receptor, CB2, is primarily found throughout the peripheral nervous system (PNS), with smaller quantities found throughout the immune system, the gastrointestinal system, as well as the aforementioned central nervous system. 
Because CB1 receptors are located primarily in the brain, they are responsible for the intoxicating effects of cannabinoids, whereas CB2 receptors are responsible for many of cannabinoids’ other effects in the body.
Δ9-THC is psychoactive because it fully binds and activates the CB1 receptor. In addition, it fully binds and activates the CB2 receptor, leading to a number of other benefits that can be had from THC intake.
By comparison, CBD will bind to both the CB1 and CB2 receptors, but does not exhibit a strong binding affinity to them. In the case of the CB1 receptor, this leads to CBD providing some neurological benefits with no psychoactive effects.