Double blinding is a method used in clinical trials to reduce the risk of bias, which can be caused intentionally or unintentionally when trial participants and/or researchers are aware of which participants are receiving which treatment (or placebo).
For example, in a trial with one treatment group and one placebo group, blinding means that the participants do not know which group they have been assigned to. In a double blind trial, neither the research team nor the participants know which participant is assigned to which group.
Sometimes the term 'single blind' is used to describe studies in which the participants are unaware of which group they are in but the research time is aware.