I think people who mock those phrases mercilessly probably never heard them used in context. They're government contractor-speak that is probably popular around the DoD, but they're not meaningless. Here, let me try.

Known known: There is a bug open in the tracker which reports that there is a race condition under certain circumstances. You have someone assigned to look into it.

Unknown known: One of the programmers noticed a race condition under certain circumstances, but he did not put it in the tracker, and as a result the information is compartmentalized and the issue is not being dealt with.

Known unknown: In preparation for your release you've got a load balancer, four application servers, and memcached set up, but you're not sure whether it will be able to handle a Slashdot on release day. You resolve to test this, when the schedule permits.

Unknown unknown: Your password recovery form permits a SQL injection attack and nobody in your organization has a clue.

The manager take away point: it is far better to have known unknowns than unknown unknowns because you can take steps to mitigate the risk, and unknown knowns are almost useless to you as a manager because they prevent you from mitigating the risk even if you're aware of a solution at some level of the organization.