You will need:

1. text domain: this is usually the file name of the language file, for example geodirectory, geodirlocation etc

   

2. exact default string: it is best to copy this direct from the language file in the .po file in the language file

   

Example code snippet 1

add_filter( 'gettext', '_my_string_changes', 10, 3);
function _my_string_changes($translation, $text, $domain ){
	if( $domain == 'geodirectory' && $text == 'Post received, your listing may need to be reviewed before going live, you can preview it %shere%s.' ){
		$translation = 'Thanks for adding a listing this will be approved within 24 hours.';
	}
	return $translation;
}

Example code snippet 2

add_filter( 'gettext', '_my_string_changes', 10, 3);
function _my_string_changes($translation, $text, $domain ){
	if( $domain == 'geodirlocation' && $text == 'Click on above field and type to filter list or add a new city.' ){
		$translation = 'Select a city or town.';
	}
	return $translation;
}