Pretty much the "forgiveness" of the ski. An expert ski will be torsionally stiffer, and require you to be a bit more on top of it to get it to perform. Think of it by comparing a typical American sedan with a high-performance European sports car -- the latter will have much tighter tolerances, and handle more astutely when driven by an accomplished driver on a twisty road, etc. The sedan will be more comfortable to drive, but will not handle as effectively when driven on curves at high speed.
That's the best layperson's analogy that I can come up with.