“Binning” is when you combine the signal from adjacent pixels in a camera, which reduces the resolution BUT improves the signal quality. Binning reduces the number of pixels in the final image, but with modern CMOS cameras this isn’t a problem – in fact we often have a wealth of pixels anyway…
So you’re basically trading spacial resolution for a better signal-to-noise ratio, which is a good thing for deepsky imaging because it reduces the number of frames you need to stack to get rid of noise in your image.
All CMOS cameras use software binning. They can’t do “hardware” binning in the same way a CCD camera can – by adding the electrical charges on a pixel up, before analogue-to-digital-conversion.
Altair GPCAM, Hypercam CMOS cameras use averaging when binning, rather than adding the binned pixel values like a CCD.
CMOS-style binning reduces the noise level with averaging rather than increasing relative pixel brightness like addition does (both techniques give the same increase in signal-to-noise ratio, which is the real goal).
Some might say additive pixel brightness increase is an important feature of CCD binning. Brightness however is not that important in itself, because it’s really just a number (a value per pixel) and it can be fixed post-processing during stretching of the image, or increased proportionally until a dim image feature appears.
The fact that CMOS cameras use “averaging” means that it’s often a good idea to turn up the gain more when you use binning to brighten the image. Since the noise has already been reduced by binning, it will be easier to process the image than if you turn up the gain the same amount without binning.
Read this post by RobinG (SharpCap author), for a more detailed mathematical explanation of CMOS Camera Binning…