A “dark frame” is shot in the dark (so e.g. covering your camera) with the same exposure as your light frames (e.g. 180sec, 300sec, 600sec) and contains the dark signal (and the so called “thermal noise”, if not -properly- cooled). Those frames allow you to remove the sensor noise from your light frames, but also to tackle pixel errors (hot and cold pixels). The combination/stack of multiple dark frames is called a “Master Dark“.