When you fire a round, burning gas builds up pressure behind the bullet and thrusts it down your bore. When the bullet leaves, a burst of host gas exists behind it, releasing a loud noise.
In comes a silencer. A silencer slows down the gas by redirecting it through a series of baffles before it can escape the silencer. This allows any unburnt powder to finish burning off, which reduces muzzle flash. It also gives the gas more time to slow down and cool, so that when it does escape the silencer, it’s moving slower and under less pressure, which means it’s also quieter.
If you want to learn more about how silencers work and get into the nitty gritty, check out our article, “An Inside Look at How Suppressors Work”.