It's exactly what the name suggests. Something is switched when the voltage across it is zero.
You usually find the term used in power switching applications, consider that in AC or rectified AC circuits the voltage is constantly changing. If you switched the power at peak voltage you risk arcing or large current surges, if you switch it at a zero point there is no voltage and probably no current so it's much easier. All AC and many DC circuits will have a zero point at least once per cycle.
Brian.