An ideal camera sensor would convert a known amount of light into an exactly predictable output voltage. Unfortunately, ideal sensors (like all other electronic devices) do not exist. Due to temperature conditions, electronic interference, etc., sensors will not convert light 100% precisely. Sometimes, the output voltage will be a bit bigger than expected and sometimes, it will be a bit smaller. The difference between the ideal signal that you expect and the real-world signal that you actually see is usually called noise. The relationship between signal and noise is called the signal-to-nose ratio (SNR).
Signal-to-noise ratio is commonly expressed as a factor such as 20 to 1, 30 to 1, etc. Signal-to-noise ratio is also frequently stated in decibels (dB). The formula for calculating a signal-to-noise ratio in dB is: SNR = 20 x log (Signal/Noise).
Once noise has become part of a signal, it can't be filtered or reduced. So it is a good idea to take precautions to reduce noise generation such as:
Using good quality sensors and electronic devices in your camera
Using a good electronic architecture when designing your camera
Lowering the temperature of the sensor and the other analog devices in your camera
Taking precautions to prevent noisy environmental conditions from influencing the signal (such as using shielded cable)
Many times, camera users will increase the gain setting on their cameras and think that they are improving signal-to-noise ratio. Actually, since increasing gain increases both the signal and the noise, the signal-to-noise ratio does not change significantly when gain is increased. Gain is not an effective tool for increasing the amount of information contained in your signal. Gain only changes the contrast of an existing image.