It's basically different flavors of random generators set up in a cartesian graph where x, and y get you the random value.
Except for the 3d, which gives you the z.
Think of like placing pixels on the canvas. The values are generated, and you go to the position to get values between 0, and 1.
Like the static on an old tv, and the flavors(Voronoi is so cool!) give you variations of that.
Of course a real detailed reference is still needed, but that could get you started.