This event is signalled by p_get_instructions() in inc/parserutils.php immediately before DokuWiki hands the text to the parser for processing, allowing the event handler to inspect or modify the data before it is parsed.
Note that modifying the data might change section positions, which in turn would break section editing if not fixed in PARSER_HANDLER_DONE.
$data contains the raw wiki text.
The following plugins are known to handle this event and their source code may be a good start for understanding and implementing a handler yourself.