Each pixel is a well that fills with electrons as photons strike it (the photoelectric effect). At the end of an exposure the charge in every well is read out and digitized by an analog-to-digital converter (ADC) into a number (ADU). Key properties: quantum efficiency (fraction of photons converted), full-well capacity (before saturation), read noise (added each readout), and bit depth (ADC levels). These set how faint a signal you can record and how cleanly.