So, a refractometer has a prism inside of it, and when you place a liquid on the glass, light goes through the liquid, slowing down and "bending" do to the properties of the particular liquid. When you look through the end of the meter, there is a small scale that tells you what the salinity of the water is, based on how much it bent. 1.026 salinity will bend more than than 1.024, and therefore reach a higher point on the scale.
The calibration screw adjusts the prism to a baseline to ensure that all of you measurements are correct. you can use distilled water to calibrate (it will measure at a fixed point every time, the instructions will tell you what), or you can buy calibration fluid, such as the stuff I have that is 1.026. I simply place a few drops on the glass, close the cover and make sure I don't have a bunch of air trapped, and then take a glance. If it's not seeing 1.026, I adjust the screw until it does. Then I know my water measurements are correct.
