Behold the simplified scheme of visible colors:
Actually, these are just their brightest levels.
Imagine, there are also darker gradations of it.
Monitors and displays work with emitting light,
coming from vast arrays of small light diodes:
(Spit a bit on your screen, and you'll see them !)
To show images, they use three base colors: RGB.
The more you mix them, the brighter a color gets:
Now printing requires inks, which are the opposite.
Millions of tiny ink dots compose an image, like this:
Inks block light, so the more inks, the darker it gets.
They are the opposite of RGB, or in between: CMY:
There you have it.
That "K" in CMYK is a method to save redundant ink.
It stands for black ink, replacing an equal mix of CMY.
Just like you use a white light, in stead of R, G, and B.
Beware, CMY inks can't be as bright as RGB light !
So you lose some brilliance, and the brightest whites:
Only special inks (like Pantone) can compensate this.
But let's not get into these more complex matters...
And don't be shocked when you see this stuff:
That's just the more scientific shape for the scheme,
showing the ranges of certain devices (e.g. printers).
Hope this helps !
More info: Color space - Wikipedia
Why is RGB generally used for computer monitors and CMYK used for printing? "