The CS Grenade is a non-lethal weapon which emits a yellow cloud of CS gas, commonly known as tear gas, which is used to temporary incapacitate its target without killing them. Enemies inside the gas will not be able to fire their weapons, and will be forced to move out of the gas cloud.
CS gas currently does not affect rangers. (However they will cough uncontrollably which does not have any effect in game)
The CS grenade can be useful for escaping enemy contact in patrols without killing them, however they can also be useful for luring out enemies in defensive positions such as trenches, possibly to allow you to slip by.