What is an ash catcher? A small glass chamber that sits on the joint of your water pipe (between the bowl slide and the pipe itself). It's filled with water, and may even include some extra percolators. The primary purpose of an ash catcher is to catch loose debris before it ends up inside your bong.
The central idea behind ash catchers is simple. The piece acts as a removable barrier and an extra layer of diffusion for your water pipe. For instance, if your rip the GRAV 12" Straight Tube Flare Bong without an ash catcher, you’ll likely end up with herb ash in your piece. But if you use an ash catcher, such as the Boo Glass 8-Arm Colored Tree Perc Ashcatcher, your hits will be smooth and ash-free.
Not all ash catchers are the same. The attachment can feature different percolators to diffuse and filter the smoke before it hits the main chamber and percolator of your pipe. Because of this, you should know the size and gender of your water pipe before purchasing an ash catcher.