We have therefore attempted to provide here a brief description of most of the basic concepts behind the survey, such that anyone with a good understanding of physics will understand. Under certain conditions, like those in the nucleus of a QSO, all atoms can be made to emit light at particular wavelengths. This is only a representative sample. There are many more lines. If we see features in an object's spectrum at precisely these wavelengths, we know that Hydrogen must be present in that object.
Light is an electro-magnetic wave, just like the signal emitted by police radar speed cameras.
We can measure the speed with which a QSO is moving away from us in almost the same way as a radar gun measures the speed at which you are approaching it. Light, sound or indeed any wave is perceived differently when it comes from a moving source. If that source is approaching the observer, the wave is squashed up onto itself making the wavelength shorter. For light, this makes it bluer. For a receeding source, light gets redder and sounds get deeper.