We see colours because there are three kinds of colour receptor in the human eye. For simplicity, I refer to these as the red, green and blue cones. Each of these three kinds responds differently to light of a particular wavelength.
For example, light in the middle of the orange region of the spectrum will induce a greater response from the red and green cones than from the blue cones, as indicated in the left half of illustration. Exactly the same response could be induced by the light with a suitable mixture of other wavelengths. In particular, the orange colour could be produced by the correct mixture of red, green and blue light, as indicated in the right half of illustration.
We can make use of the fact that there are three types of cone with their distinct response curves—we can produce a great range of the colours with only three colours of light. This is precisely what is happening right now on this computer monitor—all the colours you see here are produced from RGB (red, green and blue) pixels.
Even with three perfect RGB light sources, it would be impossible to produce every colour because of the overlapping response curves for the red and green cones. In practice, every method of producing colours has a more or less limited range, its colour gamut.
Red, green and blue are called the additive primaries because we add light from these sources to make new colours. Alternatively we could use their complementary colours, the subtractive primaries, to make other colours by selectively removing colour from white light.