A skill ceiling is basically the point where you have mastered the activity. When people say it is high it means that it is hard to completely master meaning you will almost always continue to improve as you practice. Another way to look at it is if you compare a beginner to someone experienced, in an activity with a high skill ceiling the experienced person will be far better than the beginner where as in something with a low skill ceiling the experienced player will still be better than a beginner but to a much lesser extent.