take loads of images, add them up, the signal is always the same so adds up, the noise is random so averages out!
Not quite as simple as that - the noise comes from three sources:
1) quantum noise from the signal - Poisson distribution
2) thermal noise from the sensor - approximately normal distribution
3) electronic noise from the amplifiers - approximately normal distribution
The way the composite adds up is not simple but the basic truth is that the signal to noise ratio increases with the square root of the number of frames added.
It's still best to take subs as long as possible because this minimises the contribution from amp noise, which is independent of the exposure length.
