A blindfold is a strip of fabric that's tied across your eyes so you can't see. At a birthday party, you might wear a blindfold and attempt to pin a paper tail on a picture of a donkey.
A bank robber might blindfold a hostage, and a kindergarten teacher might also tie a blindfold on the birthday boy during a party game. In both cases, the blindfold makes the person unable to see what's going on. The earliest, 14th century version of the word was blindfelled, the past participle of blindfellan, "cover the eyes," from the Old English blindfellian, "to strike blind."