The QandA episode this statement was made in http://www.abc.net.au/tv/qanda/txt/s2721753.htm
This article explains why the changes should not be considered retrospective.
The implication of the article is that Bill Shorten's argument is that the changes are retrospective in that they apply to interest on money that is already in accounts. Those accounts were created under the assumption that the money in them would receive a certain tax treatment. This is changing the tax treatment going forward.
The article says that the changes should not be considered retrospective, as they only apply to future income. It gives a legal definition of retrospective that looks correct.
Note however that Shorten's policy may still be a good one. It is quite possible that this kind of change should be opposed, even if it is not technically a retrospective change. It is quite possible that he is simply describing the problem incorrectly. You'll have to make up your own mind on that.
The basic thrust of his argument is that people contributed money to a superannuation account under the understanding that it would receive a certain tax treatment. Those people should not now get a different tax treatment in his opinion. They should get the tax treatment promised when they invested.