Light is the very narrow band of the electromagnetic spectrum (between Ultra Violet and Infra Red) which we are able to see, and the visible light spectrum has wavelengths from approximately 400 to 700 Nanometres (millionths of a millimetre). We perceive these different wavelengths of light as colours.
The human eye has two types of photoreceptors- rods and cones. Rods are sensitive in low light conditions and populate the peripheral retina. Cones are found within the macular area, and are concentrated at the fovea at the centre of vision- when we look at an object, we rotate the eye so that the image lands on the fovea. Cones function at higher levels of brightness than the rods.
Cones can be further subdivided into 3 types, Red, Green and Blue (More correctly called L,M,S- for Long, Medium and Short wavelengths). The cones will be maximally stimulated when light of the correct wavelength hits them.
Normal colour vision requires that all three cone types function correctly. A person with normal colour perception (a Trichromat) can correctly match any spectral colour with a mixture of red, green and blue light- that is why a T.V. works- it emits only three colours, but we perceive the full spectrum of colours by their mixture.
A colour normal person (a Trichromat) can match any spectral colour (a single wavelength colour which is not achieved by mixing other colours) with a mix of the 3 primary colours- red blue & green, a colour deficient will match those colours differently.