A suppressor works in much the same way as the muffler on a gas combustion engine. In fact, both were developed around the same time by Hiram Maxim. The primary function is to cool and slow the escape of hot, high pressure, combustion gasses into the surrounding atmosphere. As a practical example of how this works, imagine two inflated balloons. If you were to pop one of the balloons with a pin, the air inside would rapidly expand into the surrounding, lower pressure air over a very short time period. The result would be a loud pop. Take the second balloon and simply release the neck. It will putter around through the air as the higher pressure air inside slowly escapes through the neck into the lower pressure surrounding air making much less noise.