Silicone grease intended for application onto the filling probe. This would be to firstly, lubricate the o-rings on the filling probe itself and secondly, to allow the silicone oil to traverse into the air reservoir, where, as the shots are fired, this oil will move through the regulator (if applicable) and thereafter, throughther air port and lubricate all seals that it passes. Whatever does not deposit onto the seals / cling to the metal interior of your rifle, will be wasted and shot out the barrel / muzzle, greasing the leading inside the barrel, which might put your shots off. Therefore, use only in sparse amounts and not w/ each fill.
Recommended usually would be two drops per ten fills.
Using this product will ensure that your otherwise hard to reach seals are well lubricated and prevents unexpected downtime on your rifle due to something like an internal O-ring rupturing.