A gas piston is part of the internal mechanism used in gas operated weapons to extract spent cases and chamber a new round ready for firing.
In weapons such as the SLR or AK-47 some of the high-pressure gas produced by the round’s propellant is tapped off through a small hole in the barrel, called a gas port. The gas pushes on the head of the gas piston and forces it, and its attached rod, towards the rear of the weapon, activating the extraction mechanism as it does so. The rod and piston are then forced forward by a spring in the butt of the weapon, forcing the breach block forward again and so chambering a new round and cocking the weapon.
While gas operated weapons are generally very reliable a build up of propellant soot on the piston head can result in stoppages. Cleaning the head can be difficult and time-consuming. Abrasives should not be used as these wear down the head.