In short, the air cylinder replaces the lube reservoir pressure spring and screw. With the air cylinder, you don't have to turn the screw to maintain pressure on the lube. Constant pressure is maintained by the air cylinder (which has a pressure regulator). You need an supply of pressurized air, either a compressor, or charged air cylinder.
I don't have one yet, but I have heard many say that it's a worthwhile add on for a Star Lubesizer.