$__MAPPING = array(
	    _ACTION_FORMS => array(
        'stdform' => array(
            _TYPE => 'ActionForm'
        )
    ),
	    _ACTION_MAPPINGS => array(
        'main' => array(
            _TYPE => 'MainAction',
            _NAME => 'stdform',
            _INPUT => 'index.php',
            _VALIDATE => 0,
        ),
        'blub' => array(
            _TYPE => 'MainAction',
            _NAME => 'stdform',
            _INPUT => 'index.php',
            _VALIDATE => 0,
        ),
        'test' => array(
            _TYPE => 'MainAction',
            _NAME => 'stdform',
            _INPUT => 'index.php',
            _VALIDATE => 0,
        ),
    )
);

what does _INPUT means? like how we knoe which file to put?

That really depends on the rest of the script and what its used for.

Well, we don't know what exactly it means until we see the rest, like cptnwinky said.

I can say though that all the _<NAME>'s there are define()'d somewhere, unless the author is getting lots of errors

it's not errors though. the script is a mapping script.