The round is slower (subsonic) and therefore penetrates armour less effectively. however, it is quieter (no sonic boom, however small) and so is ideal for sniper rifles when stealth is a priority.

Means slower than the speed of sound. Unlike regular rounds, subsonic will be much quieter, as there is no sonic boom from the round breaking the sound barrier. Still have good stopping power on a decent sized pistol with subsonics, just don't try and shoot through body armor with one!

Subsonic rounds are bullets that travel less than the speed of sound. This means the sonic signature is less, you don't hear the bullet itself make the normal crack-bang when it breaks the speed of sound. It is normally used with a sound suppressor, commonly known as a silencer, which is an incorrect term. The suppressor stops the sonic signature of the gasses pushing the bullet which too are super sonic. These two methods together make a very quiet combination. The loudest sound would probably now be the bolt of the weapon if it is an automatic. Therefore the next step is to block the gas from recycling the bolt. This is not necessary with a bolt action rifle. Revolvers cannot be used with a suppressor contrary to what Hollywood has shown us. The suppressor also covers the flash from the gasses so not only is it much quieter but you do not see the normal flash from the muzzle. Some weapons cannot fire this type of round because it will not have enough gas to cycle the bolt. Some weapons work from blowback like the MP-5 SMG and almost all automatic pistols. This does not use gas but the force of the round going off. There could be a lock put on a weapon like this so that the bolt does not cycle. Some blowback weapons need a lighter bolt otherwise the subsonic round will not have enough energy to cycle the weapon. These rounds can be used for hunting so that the prey does not even flinch at the sound of the bullet. But it's mostly used in the military to take out the enemy as quietly as possible.

Most handguns fire either just below the speed of sound or just slightly higher. Subsonic will be just slightly below.