jQuery's ready is an event which gets triggered automatically when DOM is ready while holdReady is a signal/flag to hold this triggering.
holdReady was included in 1.6 version. It will work only when used before the execution or triggering of document ready event. Once ready event is fired, it has nothing to do and useless.
holdReady is useful in dynamically loading scripts before the document ready code block starts executing. Document ready code block execution is stops until holdReady release ready event execution. It holds the boolean value to hold or release the ready event.