The receptors (CB1 & CB2)
The illustration shows the cell wall and what happens in the cell after the contact with the agonist (cannabinoid). THC molecules have a high affinity (attraction) for CB1 and CB2 receptors that are located on the cell membrane (lipid layer).
This means that THC is an agonist that fits perfectly with the two receptors (keyholes) and therefore are able to communicate with cells that have these receptors.
CBD has a low affinity for CB1 and CB2 and either binds to the cell directly through the fatty membrane (lipid layer), or through a Transient Receptor Potential Vanilloid 1 (TRPV1) receptor.
This illustrates that cannabinoids are entering the cells through different keyholes, depending on the type of cannabinoid. This is a simplification, as in the real world, CBD influences also how THC interacts with the CB1 and CB2 receptors.
This is in fact true for most cannabinoids and described be the “entourage effect”, which is the synergistic effect there is between all the molecules in the cannabis product.