Humans produce cannabinoids internally through an important system in our bodies called the Endocannabinoid System (ECS). Our endocannabinoids bind to cannabinoid receptors all over our bodies to help regulate those functions such as stress recovery, nervous system protection and immune system response.
We as humans have two types of receptors: CB1 and CB2. CB1 receptors are primarily located in the central nervous system, which includes the brain. CB2 receptors are found primarily in the immune system.
When cannabis is consumed, cannabinoids like THC and CBD can interact with our receptor sites like our own endocannabinoids do. It is because many of those receptor sites are located in the central nervous system that cannabis is likely to produce changes to our cognitive and/or physical states.