In astrophotography, darks refers to calibration images taken by the camera with the lens on to block light. Common practice is to take at least 15 using the same exposure and ISO or Gain the camera used for regular photographs. Taking them close to the same temperature is important as well.
The same way a long exposure collects more light to capture the faint details of nebulae, darks pick up noise from current in your sensor. In addition, some pixels are defective and can appear darker (“cold pixels”) or brighter (“hot pixels”) than neighboring pixels. Darks isolate these pixels so they can be brightened or darkened during processing to match the rest of the image. Finally, some cameras leak light and produce what is called “amp glow” that can also be corrected with darks.