The human eye, and so human perception, is tuned to the range of wavelengths of light that make up the visible spectrum and so to the corresponding spectral colours between red and violet.
The visible spectrum is the range of wavelengths of the electromagnetic spectrum that correspond with all the different colours we see in the world.
To be exact, spectral colour is a colour corresponding to a single wavelength of visible light, but in everyday terms, spectral colours are usually composed of a narrow band of adjacent wavelengths.
Because of the way the eye works, we can see all the colours of the visible spectrum when red, green and blue lights are combined at different intensities.
The RGB colour model is designed to provide the exact stimuli to the light-sensitive cone cells in the retina to illicit perception of any predetermined colour.
Mixing wavelengths of light corresponding with the RGB primaries enables the human eye to see almost any imaginable colour including colours such as magenta that are not part of the visible spectrum.