The SNR is the abbreviation of Signal to Noise Ratio. It is an important notion in astrophotography: the signal is the data of the object being photographed (e.g. a galaxy), and the noise is the unwanted data produced when imaging.
In digital photography, the noise can have different sources; the high ISO sensitivity (created by the amplification of the electric signal) and the heat of the sensor while imaging are the main ones.
In practice, the higher the SNR, the better the overall image quality. This is the reason why stacking is used in astrophotography: by stacking several subs, an average signal will be used, which will lower the noise and increase the signal.