In science, experiments are often done comparing a treated group to a control group. The treated group is the one that receives any changes, while the control group is untreated, representing normal steady state. While the same set up is used in clinical trials, doctors usually give the control group a placebo pill, a treatment that mimics the actual drug in every way but lacks the crucial active compound.
Doctors do this consciously to prevent the patients' personal biases from convoluting the data results. The goal is to ascertain a drug's effects relative to the placebo baseline. Sounds simple enough in theory, but it turns out placebo pills can actually have an effect on the patient's outcome!
Though the placebo pills shouldn't act on the patient's symptoms in any ways, some patients report getting better from these sugar pills. And it's more than just a mind trick - the placebo effect can actually induce the brain to produce pain-relieving chemicals. Researchers also found that though placebos are non-treatment pills, some actually work better than others: Complicated machines that do nothing apparently have a higher placebo effect than injections or pills that also do nothing.
So if you find yourself in a clinical trial, ask yourself if your symptoms are getting better due to an experimental drug, or if it's just due to the incredible the power of your mind.