Imagine that the yellow dots represent the "fabric of space itself" and that the Earth is the white dot in the center. All the time space is stretching at a constant rate. Earlier on a more distant galaxy emits a blue ray of light towards the Earth. This wavetrain gets stretched by the expansion of the space through which it is moving. Some time later a closer galaxy emits another ray which has the same initial wavelength. This takes less time to travel to Earth so it gets stretched less. The two rays arrive at Earth at the same time (now), but the one which came from further away has been stretched more and is redder.