The add9 chord is a major chord with the ninth tone in the scale added. If we take the C major chord as an example, it consists of C, E and G. If we add a D we have a Cadd9 chord with the notes C, E, G and D.
You must separate this chord from the Dominant ninths that are written like C9. The difference is that a dominant 9th is made by adding the ninth to a seventh chord, like C, E, G, Bb and D forms a C9. In an add9 chord the seventh is missing.
Chord progressions with add9 chord
Add9 chords aren't very common, but they truly exist in lots of songs. A very nice sequence could be found by varying Cadd9 with a G major. You could when for example add a D major and you get something like this (used in the Green Day song "Time of Your life"):
G – Cadd9 – D
Another chord progression, including Dadd9:
G – C – Dadd9 – D7/F# – G
See also Cadd2 chords.