CMYK stands for Cyan, Magenta, Yellow and Black
RGB stands for Red Green and Blue
These are both ways of creating a huge range of colours, a bit like mixing paints in school.
CMYK is for printed items and RGB is for online.
You can see the difference between using a CMYK file and an RGB file here, the first super bright image is CMYK which does not render the colour properly, the second is RGB.