One click on a Geiger counter signifies a single nuclear disintegration, but not the type of radiation released by it. This could be alpha, beta or gamma. The clicks per second can be easily translated into becquerel, and will give the rate at which the living tissue is receiving radioactive particles. The intensity of the radiation source is being measured here.
The biological effect of this, expressed in sieverts, depends on several other factors. A conversion between these units is not easy. Modern devices provide a ‘best guess’ of the sievert equivalent. Some may not detect alpha or beta radiation. Incorporating a mica window allows these particles to be measured, though calibration to sieverts becomes more challenging then.
Microsieverts (µSv) are the most common unit. American equipment may be calibrated in millirems (mrems). One millirem equals ten microsieverts. Millisieverts (mSv) may also be used; one millisievert (1000 µSv) is a dangerous dose. [100 mrem; the recommended maximum yearly exposure for the general public] As radiation is accumulative, you should leave the area as quickly as possible.
Some Geiger counters will give data on dose per hour. The average safety limit for workers in the nuclear industry is 20 mSv/year. Firefighters at the Chernobyl nuclear power plant received an average 12 Sv over their period of exposure, from which all were ill and 30 died quickly.
Radiation on food or in water is harder to measure. Dust from a nuclear incident lands on these and contaminates them. Careful calibration against background radiation and long measuring periods, up to 12 hours, are required. Although the intensity of these sources may be low, the biological effect is compounded by ingesting them. Covering food, bringing farm animals indoors and filtering water can help.
A Geiger counter will not tell you what kind of radioactive sustance is present on food. Safety limits range from only 10 becquerels per kilogram when dealing with plutonium, to 10,000 Bq/Kg for tritium or carbon-14.
The best use of a Geiger counter in a serious emergency is to find a safe place, with a tolerable level of radioactivity. You should remain under cover until the majority of the fallout has dispersed. Four days is a recommended minimum, so a reading of 10 mSv would be the upper limit.
Remember you are keeping dust out, so you are better off in a building. Make sure there is enough water. The longer you can stay there the better, as fallout will now be covering the ground. The danger comes from inhaling or ingesting these fine particles.
Good luck with survival. You might wish you’d been less hostile to wind power.