RIP means "Raster Image Processor".
A RIP is a specific software aimed at replacing the original driver of the printer. It will offer more functionality, among other the 'imbrication'- or possibility to blend several visuals on a common predefined format.
Using a RIP also enables to speed up the analysis of the file and print result, to increase the yield and quality of the printed visuals' enlargement, to generate your own linearisation or calibration files for each base paper and optimize the color management.
The RIP used to compute only with CMYK flow, but nowadays they tend to use more RGB data, which prevail in photographic application.