The colors represent the spectrum visible to the human eye. The triangles (or triangle like) shapes represent the range of colors available to that particular color gamut.
Note: Due to the fact that monitors cannot display the full range of color visible to the eye (they are limited to sRGB) and the fact that color spaces are truly 3-dimensional shapes, these are not entirely accurate. These images are only meant to give you a rough visual idea of any particular gamut.
For more color gamuts download the RGB Color Space Visualizer Spreadsheet from Hutcheson Consulting
Images made with efg Labs Chromaticity