RP means Real Photographic. The difference between a Real Photographic postcard (RPPC) and a printed one, is easily seen under magnification - as I have illustrated below using New Brighton Lighthouse as the subject. A small 10x magnifier (which folds up, and can be kept with you while visiting fairs, antique shops etc) can be bought from any good stamp accessories supplier or camera shop. A Real Photo postcard can either have a glossy or matt surface.
Picture at left is RP - subtle gradations, no dots or patterns; picture at right is printed - made up of dots