Light Frames are your regular image exposures. Light frames contain the image you are trying to capture. The number of light frames you capture depends on many factors, but often it is a several hundred.
Dark Frames are exposed at about the same time as the Light Frames (usually immediately before or immediately after) and with exactly the same exposure settings (ISO and duration) but with the lens cap on. Dark Frames contain noise that is consistent across exposures. When using version 1.3 and earlier of Starry Sky Stacker I recommend against the use of dark frames. This is because these versions of Starry Sky Stacker did traditional dark frame subtraction and with traditional dark frame subtraction you need a minimum of 5x as many dark frames as light frames (10x would be better). Version 1.4 of Starry Sky Stacker performs some novel processing on the dark frames to extract hot pixel and background glow without getting shot noise. Therefore, with some low-noise cameras, as few as 5 dark frames may be enough and most users will be happy with 10 dark frames. With Starry Sky Stacker version 1.4 I use 10 dark frames with my Canon 5Ds (not a low-noise camera). Here are instructions for preparing master dark frames.
Flat-Field Frames can be captured any time. They are captured with the lens settings (focal length, focus and aperture) and the ISO exactly as set for your light frames, but with the lens pointed at something that is perfectly evenly illuminated. If your lens were perfect, your flat frames would have the identical value at every pixel. The flat frames can be used to correct for vignetting and some errors in the sensor. If you are happy with the vignetting corrections provided by your raw converter then flat-field frames may not be worthwhile for you. Here are instructions on preparing master flat-field frames.