Blinding is when the researchers don’t tell which group people are in. It’s done to avoid any intentional or unconscious bias in how the study is carried out. There are two kinds of blinding:
Participant Blinding where the people getting treatment in the study don’t know which treatment group they are in.
Researcher Blinding where the people running the study don’t know which group individual participants are in.
It’s not always possible to have either kind of blinding. For example, pretend you were doing a study on circumcision, it would not possible for the people involved to not know if they were circumcised or not. But you might be able to blind study researchers who analyze the data by only giving them dad where groups are labeled with generic titles. You might be able to blind in person data collection that happens after recovery from the surgery.
Sometimes researchers go to great lengths to blind studies. I found this podcast on placebos, especially the details on boule blinding studies on knee surgeries, to be fascinating.