Double blind in a trial refers to who knows what during the study. This means that both the patient and the researcher administering the drug, device, etc. do not know if what is being adminstered is the actual product or a placebo. This is done to eliminate any bias the researcher might have to sway the results to meet their hypothesis.
Runner, this is a serious question and one I never quite understood. If a patient is in serious medical need of treatment, why would he enroll himself in a study where he has a 50/50 shot of being given a placebo and not getting the real treatment that he needs? He's gambling with his life! And yet every study manages to enroll people willing to do it.