It says that jobs lost because of intoduction of labour saving machine are regained at later time thanks to increased demand caused by more efficient supply.
But what if we reached a point where one labour saving machine is replaced with even better labour saving machine before it had a chance to redeem lost jobs?
I anticipate transition of jobs and money toward trivial things. In 10 years John will cut hair of James for 1000$ and James will groom dog of John for 1000$. In the meantime theyll both enjoy nearly magical devices they bought for 10$ because they were made by robots.
http://www.fee.org/pdf/books/Economics_in_one_lesson.pdf