I think she means a happy employee is typically more productive than his or her unhappy counterpart.
When I was in delivery, one way to make sure they ended up with terrible numbers on whatever route they put me, was to tick me off before I left the building with some sarcastic comments. No air and ground together, followed ALL methods. Sorted continuously and walked with a safe pace to each house. Happy employees don't really hose the boss as much as an unhappy one