Starguide - A Practical Guide to Astro-Photography

In the context of image processing, binning is the procedure of combining a cluster of pixels into a single pixel. As such, in 2x2 binning, an array of 4 pixels becomes a single larger pixel, reducing the overall number of pixels. 

This aggregation, although associated with loss of information, reduces the amount of data to be processed, facilitating the analysis. For instance, binning the data may also reduce the impact of read noise on the processed image (at the cost of a lower resolution). 

ZWO Optical tutorial about binning

Created with the Personal Edition of HelpNDoc: Free EPub producer