A placebo is a dummy treatment with no active ingredient, such as a sugar pill, used in research so a real treatment can be compared against it.
If people taking a medicine improve more than people taking a placebo, the difference can be credited to the medicine. Participants don't know which they are taking. In a double-blind trial the researchers don't know either.
The placebo effect is real. People often improve on a placebo, because expectation, attention and the passage of time all affect how someone feels. That doesn't mean the original difficulty was made up.