From B2evolution

A Renderer Plugin is a Plugin like all others, but it's called so, because it does render items/posts.

It is not restricted to only render content of items, but can hook any other events also.

Rendering means, that it looks for special markup in the item's content and replaces/renders parts thereof, before the item gets displayed.

A simple example for this is the Wacko plugin and the Smilies plugin. There is also a tutorial plugin to showcase the purpose of a renderer plugin and to teach you how they work as well as how to make one, it is available [here].

The Apply Rendering setting of a plugin defines, if it gets asked to render the content of an item.