It's a dual weatherproof remote plug with rubber pad which can work with most rifle or carbine applications
It can provide remote momentary-on and constant-on weapon light activation that you can snap it on the 20mm picatinny rails without tools
There are two plugs you can use: the large one is for Element Airsoft or Night Evolution flashlight, and the other one is for Element PEQ & M3X / DBAL-EmkII / WMX200