Over-unity production refers to a metric called the Coefficient of Performance, COP. This is simply the ratio of power produced by a device compared to the power it takes to run it. For electrical devices, power is measured as watts. So, COP = watts out/watts in.
Free energy devices violate the classical laws of thermodynamics that state that you cannot get out more power out than you put in. That means that no device should be able to have a COP greater that 1. The problem is that since there are currently no ways of measuring the energy extracted from the quantum vacuum directly, there is no way to account for the energy coming from there. As a result, free energy devices will always have COP values greater than 1, or "over unity", sometimes exceeding 100 even much more.