It means that neither the experimental unit nor the researcher in contact with the experimental unit knows which treatment the experimental unit is receiving.
Work Step by Step
A person under alcohol dependence may ask for a treatment. If he/she discovers that they received a false medication, this person may become disappointed and change his/her behavior and, maybe, drink more. Thus, double-blind experiments are useful because they make it harder for the patient to figure out if it is a real treatment or not.