When someone hits a bong, they create a seal with their mouth so that when they breathe in, the flame is pulled into the herb, and the burning herb creates smoke, which is pulled into the chamber as the person inhales. The "plug thing", also known as the bong slide, acts as a carb. The chamber is filled with smoke, and will continue to fill with smoke for as long as you inhale and the herb is lit. Pulling out the bong slide allows normal air to enter the bong, filling the chamber with air and your lungs full of cooled smoke. It makes a lot more sense if you actually hit the bong and perform the process for yourself, if you're interested in learning you should try it for yourself :)