A placebo is a fake medication, say for example a sugar pill, which has no effect on the patient aside from making them believe they're consuming real medication. A placebo is offered to a patient to test findings as compared to subjects getting real medications. In one particular study in Hennepin County Medical Center, a new drug (VEGF) was being tested that was suppose to relieve chest pain. In this study, patients were rated by how long they could walk on a treadmill before suffering from chest pain.
The results showed that those who got low doses of the new drug could walk on the treadmill 26 seconds longer than before. Subjects who got high doses did even better walking an additional 32 seconds before having pain. But even with these results the study was a complete failure. It seems the subjects who got the placebo were able to walk an average of 43 seconds longer than before.