A neck weight used in two different disciplines of freediving: Dynamic apnoea and deep apnoea.
During the dynamic discipline, the freediver looks for a hydrodynamic, horizontal position. The large volume of air in the lungs creates buoyancy in the upper body. A neck weight helps maintain the correct position.
In depth, during the descent, the freediver seeks a hydrodynamic position during the "free fall". A weight at the head level helps.