As far as I have read, visible light refers to the part of the EMR spectrum that our eyes are sensitive to and the presence of all the colours of visible light at a single spot forms white light. Why, then, are digital screens able to create white just from a combination of Red, Blue and Green light waves?
White sunlight is a mixture of a huge range of frequencies (i.e. colours), of which most - but not all - are in the range that are visible to the eye. White is not one single colour, it is a mix of many colours.
The eye has 3 different colour receptors (cone cells). Each type is most sensitive to a certain colour range. Their peak wavelengths are in the red, green and blue regions of the visual spectrum, respectively. The brain then mixes the signals from all three types, to produce the sensation of a "colour".
Because our eyes evolved to see in sunlight, if the red, blue and green levels of the perceived spectrum are approximately those of sunlight, we see that colour as being the same as that of sunlight - something we call "white". Again, white is a mix of colours.
There are other perceived colours that do not appear in the spectrum. A common example is brown. Like white, brown is a mixture of pure colours. No part of the electromagnetic spectrum is brown, but our brain interprets certain mixtures of wavelengths that way. Tree trunks reflect only those parts of the sunlight that make the brain think "brown".
In the same way, if something reflects all wavelengths of sunlight, the brain thinks "white". In artificial lighting, if we mix the appropriate amounts of red, blue and green light, all three types of cone cells get the right stimulation to make the brain perceive it like sunlight, i.e. white. If the mixture is not exactly right, the brain will see it as some other shade, like pink or grey.
In fact, different "white" lights have different colour shades. It is said they have a different "colour temperature". The temperature of the sun is close to 6000K and light of that colour temperature looks like sunlight. Many white LEDs have colour temperatures of around 9000K, and their light will have a bluish tinge. Incandescents globes are closer to 4000K and look reddish - but we call all of these lightsources "white".
White is not a color. By that, I mean there is not one wavelength of light corresponding to what we call "white color". Instead, white light is what we comprehend as the combination of many colors, namely all the colors of visible spectrum. On the other hand, red, green and blue are well defined colors meaning that they are monochromatic light, i.e. they have one specific wavelength. From what I understand, the RGB are at the proper positions in the spectrum and have enough deviation to neighboring wavelengths so that the resulting combination is approximately white light.
The eye typically has a number of different "receivers" (not a biologist) that respond differently the different wavelengths, by transmitting for example both red and green colours, the eye measures both and adds them together in the brain, resulting in another colour.
I hope this answered your question.