When it comes to computer hardware, cooler is always better. It is highly unlikely that you will ever have to worry about minimum safe temperatures unless you are working in an extreme environment, such as outer space, the ocean floor or Antarctica. Maximum temperatures vary by component. Check the technical specifications for your hardware for actual safe minimum and maximum temps. As an example, nVidia graphics cards are rated for a maximum operating temperature of around 105 degrees Celsius or 105 degrees Celsius.
Review the temperatures listed. The Readings tab will give you a list of internal temperatures gathered from the internal temperature sensors in your system. SpeedFan can also monitor and adjust fan speeds for fans connected directly to the motherboard. Change which temperatures are visible in the Readings tab by pressing the "Configure" button. You can select or deselect the temperatures you want to monitor.
Look at the icons next to the temperature readings. Green ticks mean that the component is running at a safe temperature. A blue down arrow means that the component is running cooler than normal or is decreasing in temperature. A red up arrow means that the temperature of that component is rising. A flame icon means that the component is running above normal temperatures.
Compare the temperature readings to the safe minimum and maximum temperatures for your hardware. If the hardware is running within safe parameters, you can ignore the SpeedFan icons. That said, running at temperatures near safe maximums for any length of time can shorten the life of your components.
Compare temperatures over a length of time with the Charts tab. Select the component you want to chart from the list in the pane below the black chart area. This will give you a good idea of the range of temperatures your hardware experiences over time, which is a better measurement than static, single-moment readings.
External temperature will influence your computer's internal temperatures. This is why server rooms are kept so cold. Clean your fans regularly. Dust build up will reduce airflow and increase temperatures. Buy new fans that run faster or more efficiently to improve cooling. You can change SpeedFan's temperature readings from Celsius to Fahrenheit. Go to the "Options" tab in the "Configure" menu and select "Celsius" or "Fahrenheit."
Do not use SpeedFan to alter the speeds of your fans unless you know what you are doing. Decreasing fan speed can raise internal temperatures, possibly to unsafe levels. Setting the fans, via SpeedFan, to static RPMs can prevent your hardware from compensating for increased load -- and thus increased temp -- which can lead to overheating.