I think that description is more than a little better.
> Mapnik raster map rendering engine
An engine that renders maps as raster images, called Mapnik. Should be clear from the quote.
> CartoCSS styling language
A CSS like language to style maps (change the looks of) instead of HTML. If you know CSS, that should follow from the description.
> It was originally developed by Mapbox, but they've since moved to vector rendering.
Should be pretty obvious from context that Mapbox is the name of the company that originally developed the software.
ELI5: It is for drawing pictures of maps.