A ball head is a metal or plastic apparatus placed on top of a tripod

Tripod

A tripod is a portable three-legged frame, used as a platform for supporting the weight and maintaining the stability of some other object. The word comes from the Greek tripous, meaning "three feet". A tripod provides stability against downward forces, horizontal forces and moments about the...

that increases stability and provides faster, more accurate rotation for the photographer. They are also lighter than traditional three-way pan-tilt tripod heads. With fewer parts and a much simpler mechanism, ball heads are usually preferred by more advanced photographers. However, they are usually quite expensive due to the precision-made pieces inside and the detailed finish outside -- most are more than $200 USD.