Attaching a Suppressor to a weapon decreases that weapon's muzzle flash and audio range at the cost of bullet velocity. Firing a weapon with an attached suppressor does not cause the player to appear on the mini-map.
The reduced bullet velocity increases bullet drop and increases the distances by which you have to lead your target. This makes the suppressor an attachment that is not well suited for long range engagements.
In the original PlanetSide, the Suppressor was a common pool SMG. You would spawn with a Suppressor after each death.