The entourage effect describes the phenomenon where the 400+ compounds in cannabis work in concert to have a synergistic effect on the body. In other words, it is the sum of its parts that make cannabis so effective.
For example, 100mg of isolated CBD may be substantially less effective at alleviating symptoms than 100mg of a whole-plant or full spectrum, cannabis extract.
Vitamin C may be a good analogy. Many vitamin C supplements are presented in the form of ascorbic acid. Ascorbic acid is just one of the many compounds that comprise the whole vitamin C. In order to be most effective, vitamin C must also exist alongside rutin, Factor K, Factor J, Factor P, Tyrosinase, and Ascorbigen. In fact, in all of his years of research, the discoverer of Vitamin C and Nobel prize laureate Dr. Albert Szent-Georgi was never able to cure scurvy with isolated ascorbic acid. In contrast, eating an orange provides whole vitamin C with all the necessary cofactors for optimum absorption.
It is this very concept that lies at the center of the CBD oil from hemp vs. CBD oil from cannabis debate.
While it may be cheaper and more cost-effective to extract CBD from industrial hemp, users may ultimately experience fewer benefits due to the absence of clinically significant levels of terpenes and other compounds which occur in abundance in high-CBD cannabis.
Agricultural hemp is much closer to the kind of cannabis that one would find growing naturally, whereas high-CBD cannabis is hybridized and altered by growers to produce the highest levels of the desired compounds.
Unfortunately, there are no hard facts about the validity of the entourage effect. It is up to each individual to research, consult with medical professionals, and decide for themselves the best path forward.