This item is an adjustable weight system for running training (sled simulation), frequently used by athletes and sprinters to build explosive power and endurance through additional pulling resistance. 

The main components visible in the image include:

The outer shell (sled element): A larger black PVC covering equipped with slots and robust hook-and-loop fasteners. 

Removable weight pockets: The inner transparent-black compartments designed to keep filled sandbags securely in place even during intense movements. 

Adjustable pull strap and hip belt: The long black nylon strap that connects the weight bag being pulled to the runner’s hip belt.