Chord tones are the notes that make up the chord currently sounding. In a C major chord, C, E, and G are the chord tones. Using these notes in a melody makes it sit stably with the accompaniment.
The basic idea
Even among the notes of a scale, a note sounds different depending on whether it is a chord tone. Chord tones tend to work as landing points, while the other notes act as passing notes or as color.
Its role in writing and performing
Placing long notes and phrase endings on chord tones makes the relationship with the chord progression clear. Changing the note you aim for as the chord changes lets you write melodies that follow the accompaniment.
How to try it in a DAW
Display the MIDI notes of the chord and place a melody above them to see which notes overlap. Writing a short melody with chord tones only, then adding other notes in between, makes the difference easy to hear.
How it differs from related terms
Notes other than chord tones, whether inside or outside the scale, work as non-chord tones that create tension and movement. Rather than using only one or the other, move between them.
If a melody does not seem to fit the chords, check whether you have placed chord tones at the important positions.
