When a gun fires a bullet, the bullet breaks the sound barrier and creates a sonic boom sound. A gun that fires a bullet which breaks the sound barrier is impossible to suppress without slowing down the bullet.
The way a gun fires a bullet is by creating enough pressure behind the bullet to push it out of the barrel. There are very hot gasses which expand very quickly. A suppressor (also known as a silencer) slows the expansion of the gas outside of the barrel. If you were to disassemble a suppressor, you would see it is basically a tube with small holes that feed into another chamber. By giving the gasses a slightly larger area to expand in, it allows the gas to expand and cool at a slower rate.