Jim: I like the new quarterback on our high school football team this year.
Steve: Yeah, me too. He is a very good player and he seems like he has a good head on his shoulders.
Sara: I heard we hired a new secretary for the office.
Amy: Yes, her name is Suzie. She is a very hard worker and she has a good head on her shoulders.
Meaning: This American idiom is used to describe someone who is acts maturely and has good, sound judgement.