Ball binding is the act of tying up the scrotum of a male victim (and often penis too), usually with rope ("balls" being a slang term for testicles). It is often an addition to bondage, and is sometimes done for humiliation. If the ropes are tight and painful, it can be regarded as genitorture; care must be taken not to cut off the blood supply for a long period, or the genitals can be permanently damaged (see Advice: Genitorture).
The rope may just go around the base of the scrotum, or round each ball so as to separate them.
If the ball binding is not attached to other ropes, it does not in itself contribute to the bondage. However, it may be attached to a crotch rope joined also to bonds holding the wrists behind the victim's back as a way to restrain the arms.