An ash catcher is a smoking accessory used with water pipes or glass bongs to enhance filtration and prevent ash and debris from entering the main chamber. It is typically attached between the bowl and the water pipe, acting as an additional filtration chamber. 

Bong ash catcher usually consists of a glass chamber with a percolator or diffuser inside. When smoke passes through the ash catcher, it undergoes further filtration, cooling, and diffusion before entering the main chamber. This helps to remove impurities, ash, and some tar, resulting in a smoother and cleaner smoking experience, no need to clean your bong as often.