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.
Photosite
The technical term for each individual light receptor on the camera sensor. The light captured by each photosite later goes on to form a single pixel in the digital image, but only after being processed by the camera and/or your photo editing software.