The endocannabinoid system is among the body's many systems––including the nervous, immune, and cardiovascular systems––that operate to keep us healthy. It's a widespread system that is involved in restoring homeostasis (normality) after pathologic insult (disease) or trauma––which in this medical context can refer to anything from an injury (bad!) to natural recovery from exercise (good!).
A functional quality all of these systems share is the need for molecules that are secreted into body fluids by one group of cells and attached to receptors on nearby cells downstream. These receptors, in turn, switch on a specific response, such as nerve transmission, or an immune attack on an invading organism, for example.
For working order to be maintained in our bodies, however, the stimulation of these receptors must be balanced by mechanisms that put on the brakes. The inadequate inhibition of stimulatory responses is present in diseases such as seizures, muscle spasms, auto-immune diseases, and even the over-exuberant immune response to influenza or COVID-19.
The endocannabinoid system is a modulatory system, which in the nervous system runs in the opposite direction of neurotransmitter-mediated nerve pulses to prevent overstimulation. When we are stressed, the number of cannabinoid receptors in our bodies actually increases, augmenting their effect. If the secretory cells can manufacture enough endocannabinoid, such as anandamide, to bind to these receptors, then they can dampen over-active responses in the system. Supplementing the internal endocannabinoids, then, may assist in restoring normalcy––or homeostasis––in the system.
Interestingly, this 'modulation' of nerve conduction can result in psychotropic effects such as the 'runner's high' or the euphoria associated with the use of THC, which might ordinarily be considered stimulatory responses. Although the mechanism of these phenomena is not fully understood, perhaps moving into a more balanced state from the stress of modern life is associated with feelings of pleasure. That the endocannabinoid system is modulatory and not directly responsible for stimulating effects is the reason that exocannabinoids such as CBD and even THC are so safe. Even at very high doses, the side effects are not life-threatening (acute overdose does not occur) and few tissues seem to be damaged from chronic administration.