RIP (Raster Image Processor) in simple words is the driver for large format printers, a data interpreter from image file into a printer-understandable form. RIP can tell the printer how to lay down dots of ink to reproduce an image on the printed page. Most RIPs available today can process raster data (bitmap or photo), vector data (design elements created in a graphics application), and text.
But modern RIPs are much more than a simple driver.  A professional RIP optimizes the performance of virtually every large or wide format device available today, in terms of color reproduction, throughput speeds, and workflow efficiency.  Most important RIP featiures are color management with ICC profiles, layout possibilities (nesting, tiling).