From Halopedia, the Halo wiki
Stabilization Jets are a set of jets attached to a firearm, to help control vertical recoil. It generally takes the form of 4 small jets placed on the barrel of the weapon.
When the user fires a weapon, stabilization jets will activate to keep the weapon facing the same direction, helping the user keep control of the weapon.
The following weapons have been known to utilize this attachment:
Another look at the jets.
Stabilization Jets on the Appetite for Destruction M739 SAW.