In case you don't know: DokuWiki parses Wiki syntax into an instruction array which then is "rendered" to the desired output format. This output format is usually the XHTML to display a nicely formatted Wiki page.
Pluggable renderers allow plugin authors to write their own renderer to output any format they want. This adds another bit of flexibility to DokuWiki.
The idea for these renderers was inspired by the S5 system (developed by Eric Meyer) that was essentially a web-based slide show (without the need for a messy client-side application). Unfortunately, the slide shows were hard to make - which is where the renderers come in. In fact, he's already created the tool to get you started - the S5 DokuWiki Plugin. Check out his example to see the results.