A placebo effect happens when a patient feels better after taking fake medicine, or when they believe they're taking medicine although they really aren't.
Scientific studies have shown that people often have a positive reaction (both physical and mental) when they think they're taking a drug or receiving a treatment — even if they're not. This placebo effect has to be accounted for in medical studies, so that researchers know a drug is really working. Often one group is given the substance being studied, and the other takes a placebo. Placebo means "I shall please" in Latin.