Hunky-Dory
A renderer is something that paints the pixels you see on your screen.
The video in encoded format does not directly contain pixels, so the video decoder has to generate the (video)pixels from the stream.
But you do not directly see this pixels, these are then fed into the renderer which transforms/blends the pixels, possible changes colors and much more.
For example the adjusting of the video to your head position is done in the renderer, also the painting of the OSD over the video.
There need not be a difference in the end result, 2 different renderers can possibly get exactly the same result.
