Noise is the digital equivalent of grain in film. It’s random electrical signals captured by the photosites on the camera sensor, and usually this background noise level is so low compared to the brightness of the captured picture itself that you just don’t notice it.
Signal to noise ratio
The 'signal' is the detail in the scene that you want to capture, while the 'noise' is the background electrical interference in the camera's capture hardware. At higher ISO settings, the image signal is weaker and has to be amplified, and this means the background noise is amplified too – so the signal to noise ratio is lower, or worse.