A blindfold is an item used to void the subject's sense of sight. A blindfold is usually a piece of material (such as tape, cloth, etc.) which is pulled over the eyes. The blindfold catches most light, making it impossible for the subject to see.
Blindfolds have many purposes, although in BDSM and spanking scenarios, they are most commonly used to give the subject a sense of helplessness. A blindfolded subject is often more confused, and is unable to anticipate what is going to happen to them (such as when the spanks are coming). A blindfolded subject is also able to put up less resistance, and often expresses more surprise to events.