The add9 chord is a major triad with an added ninth — no seventh. This gives it an open, bright, modern sound that is simpler and less jazzy than a full 9th chord. Add9 chords are staples of pop, worship, and acoustic music.
Add9 vs 9th chords: Cadd9 = C–D–E–G (no seventh). C9 = C–D–E–G–B♭ (includes the dominant seventh). The absence of the seventh makes add9 simpler and brighter — it works as a colourful substitute for any major chord.