A drop checker has fluid inside that changes colour in accordance to how much CO2 is present in the water.
Blue means the CO2 level is low
Lime green means the CO2 level is good
Yellow means the CO2 level is to high
But just remember that the fluid does take time to change to show the CO2 level, so also always keep your eye on your fishes behaviour, if they are gathering at the top of the tank and/or gasping it is likley the CO2 level is to high.
This info is only going by what I have learned in the short period of time that I have been running pressurised CO2, but we are lucky on this forum as we have access to some very good info and also people who have lots of experiance using CO2 in the aquarium.
This makes everything a lot easier to learn as 99% of the time you will get the correct advice first time.
If you are unsure of anything always ask here first.