What is the meaning of zero voltage switching (ZVS) and zero current switching (ZCS)?
That's actually pretty simple:

In many situations, you need to switch a load on and off – for example, to dim a lamp.

It's often advantageous to switch something when either the current flowing is zero, or the voltage across the load is zero.

Now, if your load is a pure resistance, then the point where zero current flows is exactly the point where zero voltage happens. 

But for things like inductive or capacitive loads, that's not the case – current and voltage are out of phase for these kinds of loads.

The motivation to do something like switching a load when the current is zero is that this means you don't get \$\frac{\partial\,i(t)}{\partial t}\rightarrow \infty\$ that you'd get when switching off something when a current is flowing. And because of the physical laws, having a high \$\frac{\partial\,i(t)}{\partial t}\$ in an inductor is very bad – because it will cause a voltage peak of a very high amplitude!