This is a standard colour gamut diagram. It shows the ranges of different colour systems. It is particularly useful in distinguishing the difference in colour range of RGB and CMYK, due to the fact that RGB is the colour system used on computers and screen based things, whereas CMYK is the system used in printing. Because not all RGB colours can be printed in CMYK, it is useful as a graphic designer to be absolutely sure you are printing the right colours. Otherwise you may have a nice bright orange on screen (I use orange as an example as it is a particularly difficult colour to print) which may end up being printed as a murky brown. It is always good to be sure. And this diagram expresses why.
The larger black triangle shows Adobe software RGB colours, the smaller black triangle shows standard RGB colour and the blob with the red outline shows what colours can actually be printed with CMYK. It is clear that greens and oranges are the most difficult to print in CMYK. Graphic designers are expected to get their colours right as a point of professional practice. It is an error that can be so grave, that it may result in a designer not being paid for the job at hand.
Be seeing you!