A non-chord tone is a note that is not part of the chord currently sounding. It is not necessarily a wrong note; it is used to add motion, tension, and character to a melody.
The basic idea
Its roles vary: a note passing between chord tones, a note that touches a neighbor for an instant, or a note that prepares the move to the next chord. Held long and loud it can sound unstable, so where you place it and where it resolves both matter.
Its role in writing and performing
Where chord tones alone would leave a melody as bare framework, adding non-chord tones creates a smoother flow. They are also related to the sound of tension chords.
How to try it in a DAW
Take a short melody built only from chord tones and add neighboring notes from the scale in between. Landing on a chord tone at the end of the phrase keeps things settled even with all that movement.
Difference from related terms
A non-chord tone is a note outside the chord, and a chord tone is a note that makes up the chord. Whether it sounds out of place depends less on the note itself than on timing, length, and its relationship to the next note.
Even a note that sounds off becomes an expressive one when you are conscious of the path it takes and where it resolves.
