With light colour is made up of RED, GREEN, and BLUE. With ink colour is made up of CYAN, MAGENTA, YELLOW, and BLACK. Hence the RGB and CMYK Acronyms. Images can be saved in both colour modes. Images saved in RGB mode are for use on the internet or viewing on screens (as screens use light to make the colour). Images saved in CMYK mode are for printing and may look a little less vibrant when viewed on a screen.
Any RGB images used within your artwork will be converted by the printer when your document is printed. This may cause a shift in colour.