A percolator bong refers to a water pipe that helps cool down the smoke before inhaling it. This is achieved by forcing the smoke right into the water, filtering it. If you hear that bubbling sound on your bong, that is the percolator doing its job.
When you put your cannabis in a bowl and light it up, you will notice the smoke being filtered right through the water, being pulled right through the tube of the bong. This is when you put your mouth over the mouthpiece, inhaling the smoke, getting stoned after getting a few smooth hits.
There are different options on percolator bongs currently available on the market these days. All of them are designed to break the flow of water up, thus helping in cooling the smoke down while it moves across the water chamber of the bong. They are usually dome-shaped though there is a wide range of designs currently available these days.
The process of filtering through a percolator bong helps in cooling and filtering the smoke in two solid ways. One is giving the smoke a different opportunity to get in contact with water, helping the smoke to cool down. Also, the bubbles are broken down into smaller ones, as a result of the pressure in the percolator.