Generally speaking the terms "an ear for music" -or- "a musical ear" allude to an individual's ability to hear chord changes (progressions.) This is a factor that is vitally important in the area of jazz, because the players are improvising melodic materials that are based upon "the changes" - as they are called.)
There are various degrees of talent involved here, because not only must a good player relate improvised materials to the chord progressions, but his playing must "swing."
To a person who has a truly sensitive harmonic ear, beautiful chord changes are nothing less than thrilling! Such a listener may not only be thrilled by beautiful harmony, but put off by players who butcher the chord changes.
Another vital element involving the chord progressions is their "voicings," which involve doublings and spacings of the tones. When a player plays wrong chords in music, he or she is said to have a "tin ear."
Here, we must be reminded that we are speaking of individuals who are playing without written music.
Many fine players are true artists, who rely on written music. Then there are those who have "tin ears," playing out of tune - sometimes with almost vulgar ardor.
One last factor regarding chord changes: Absolutely one of the most important factors in all music is the BASS LINE. The ability to hear a bass line and use it creatively along with beautiful chords is a hallmark of true musicianship!
Indeed, Sir Donald Tovey, a famous British musicologist once made a supremely insightful observation: