Critical Point refers to a specific point on an object (Infantry, Vehicle) that yields more damage than normal areas of the object when hit. When a player targets an opponent's vehicle or destructable object, the critical point is circled with an emblem that resembles the emblem shown in the reticule when you get a headshot (this emblem also shows up in the reticule when you hit a critical point). If the critical point of a vehicle is hit with a rocket launcher, the vehicle is likely to recieve much more damage, sometimes resulting in a one-hit kill. Critical points are very helpful to teams that are at a disadvantage in terms of vehicles.
Critical Point may also refer to the areas required to destroy in space battles. These would be the engines, life support, bridge, communications array, sensor array, and possibly the auto-turret module.