Jakey, a placebo is typically a medicine or substance that has no action except by what it makes the recipient/patient think it does. If I recall correctly, the term itself means "pleasing" or "it pleases."

Placebos can work just as effectively as real medicines when they work, at least in the right people. Similarly they can cause negative effects in people, too. From what I've been taught, legitimate doctors don't really prescribe any type of placebos, but they are used in pharmaceutical testing or clinical trials, where placebos are given to the control groups to scientifically test the groups that are receiving the active medicines that are being tested.

You can read all about the concept here.

Placebo - Wikipedia, the free encyclopedia

If you want some examples of placebos in action, we can make some up for you. A good cannabis-related one might be ground hemp leaf or oregano (something that doesn't have the active compounds in cannabis) in place of real cannabis bud with trichomes and THC content. In a person who who was susceptible to either positive or negative placebo effect, smoked hemp leaves might make him high as a kite, when in reality it was just the power of suggestion causing that result. As a negative placebo effect, smoking something completely cannabis free might make a person freak out or get jittery or paranoid, when it wasn't doing that at all.

Does that help?