In a normal, unsuppressed semiautomatic pistol gunshot, you pull the trigger, a precise little pin strikes the back-center of a bullet, this causes the gunpowder in the back of the bullet to spark and ignite and explode. The projectile portion of the bullet rides the wave of the explosion at supersonic speed down the barrel of the gun, which determines the direction of the path. There is an initial increase of backpressure of gas between the projectile and the back of the barrel, but semiautomatic weapons make use of this to push back the slide, expelling the spent casing and that gas and allowing the spring in the magazine to push the next round into place for the next shot, also significantly reducing recoil in the process.

Suppressors (or “silencers”) work by slowing the bullet down and altering the propulsion gas path. Subsonic speeds means no sonic boom. The downside is that you must manually cycle each round yourself, and you will likely experience more recoil per shot.

Afaik, suppressors are pretty damn hard to legally obtain, so my knowledge of them is a combination of my firsthand accounts with my unsuppressed guns, secondhand accounts of suppressors, a moderate understanding of physics, and some guesswork. I could be wrong about some of this, but this is my general understanding that I carry around with me.