Basically, they are metal threads that can be put in the ski to screw the bindings into, sort of like those on a snowboard.  They work well if you previously had a binding rip-out because they use a bigger hole than a normal binding screw does, so you can just drill out the old, stripped hole, put in heli-coils, and mount the binding as usual without drilling any new holes.  When you do this, it is very hard to rip-out the binding because of the metal threads they create.
Here's a good tutorial if that was confusing at all.