As SkiMangoJazz said:
It describes how curved the sidecut of the ski is. Lower radius = more curved / more sidecut; higher radius = less curved / less sidecut.
Think of it like this:
Set a ski down on the floor in a gym. Climb up on a tall ladder and look down at it, paying attention to one edge. Notice that it's curved. Picture the circle that's formed if you extend the curve. The radius of that circle is the "radius" we're talking about.
A few notes:
- It's approximate, for a number of reasons, most importantly: the curve of a ski's sidecut typically isn't uniform, i.e. it's not exactly an arc from a circle.
- Typically, it's estimated by considering (i) the width at three points: near the tip, at the narrowest point (aka the waist) and near the tail and (ii) the length of the ski (or, to be more accurate, the distance between the three points at which you measured the width).
- If you do a search, you'll discover numerous threads on this topic, some of them lengthy, some of them abstruse.