A diffused downstem is the part of a bong or water pipe that sits in the water and has small slits or holes at the end. Instead of smoke coming through as one big stream, those openings break it into a bunch of tiny bubbles as it passes through the water. This increases surface area, which cools the smoke and filters out some harshness, making each hit smoother and easier on your lungs compared to a solid (non-diffused) downstem. See your cashier for available styles/colors/sizes.