The turn radius of the ski determines the size of turn the ski naturally wants to make.
A turn radius of <11m is usually for a junior, young adult, or slalom race ski.
11-15m allow for shorter turns making the ski easier to turn. These are often for young adult or beginner to intermediate skiers or skis designed for on trail skiing.
16-20m allow for medium to short turns offering a very versatile feel. These are common in intermediate to advanced all mountain skis.
21-29m allow for medium to long turns making them very stable under speed. These are common in advanced to expert level skis.
30m> allow for very long turns and are usually reserved for expert level big mountain and powder skis.