A nonchord tone, nonharmonic tone, or non-harmony note is a note in a piece of music which is not a part of the chord that is formed by the other notes sounding at the time. Nonchord tones are most often discussed in the context of music of the common practice period, but can be used in analysis of other types of tonal music as well.
This is a note in the melody (or the chord) that is taking part in the transition from one chord to another. Passing tones help to create “tension” between the melody and the accompaniment that is released when the melody and the chord are “resolved.”
Good music, much like art, theater, literature, or relationships and life, for that matter, tends to be a series of tensions and resolutions (or releases) of varying intensities.