Instrument for smoking weed. Similar to a pipe, in that you fill a bowl with ground bud and inhale through the mouthpiece, drawing smoke through the glass instrument. However, bongs are large and have space to fill up with smoke, suitable for taking large inhalations of smoke. They usually have a space in the bottom for water, and a tube that draws the smoke down from the bowl, through the water, and into the chamber of the bong. In the process, the water cools and filters the smoke, making for a smoother smoking experience.
Bongs can be large and elaborate or small and precise. Blown-glass bongs are like works of art and may be collectible.
The bowl of a bong is where you put your ground plant product. You light the product with a lighter and suck in the mouthpiece as the flame is held over the bowl. The suction draws the flame down into the plant material, which turns to smoke that is drawn into the body of the bong.
The bowl can be replaced and there are lots of different options at the local Smoke Shop. You might get a different bowl because yours broke, or because you want something larger or smaller than the one you already have.
The bowl can also be replaced with a banger, which is a special type of glass bowl made to hold concentrates. Using a banger requires heating the glass bowl with a flame torch until it is red hot and then adding in concentrated resin, which melts and smokes and is then inhaled through the bong. Smoking resin through a bong is called dabbing.