A refractometer looks a lot like a monocular but has been adapted to include an angled glass surface on the rear end with a hinged cap. It works by placing your aquarium water onto the glass surface with a provided pipette and lowering the cap onto the liquid. Holding the lens to your eye you look through it towards a light source, and establish what your salinity is on an imprinted scale within the device to ensure that your saltwater aquarium salinity is not too high or too low.
There are many brands of refractometers of the market, and not all of them are used for the purpose of testing the salinity of a marine tank.
Refractometers are used for wine making, beer making, and testing sugar content.
It is very important that you spend your money on a refractometer that has been specifically produced for the saltwater aquarium trade to ensure you provide the correct salinity for your inhabitants, such as this one: