RGB is how computer monitors interpret color. The name stands for Red, Green, Blue: these are the three colors of light that are added together to produce a vast range of colors seen on the monitor. Many colors viewable in RGB cannot be achieved by printing in CMYK. This is why it's important to carefully choose colors that will print well in CMYK.
CMYK is how most printers print color. The name stands for Cyan, Magenta, Yellow, Black: these are the four color pigments that are added together to produce the range of colors available in this type of printing.
Why working in RGB causes issues: RGB on a monitor is very different than CMYK on a press. RGB color space is capable of producing many more colors than CMYK. This is why we recommend working in CMYK, and choosing your colors carefully: referring to the values that make up the color, and not just its appearance on the screen. This will help you to ensure you're choosing a value that will print much closer to your intended selection.