A person who "ma głowę na karku" is sensible and knows what they are doing, for example "On ma głowę na karku, poradzi sobie w życiu" (he'll do well in life).

On the opposite you can say: "on buja w obłokach" - "he got his head in clouds" - which means the same as english idiom. So to have "head on your neck" ("mieć głowę na karku") - instead of inside the clouds, has just the opposite meaning:)