Play Dadd9 one string at a time and you hear D, F#, A and E. D is the root, F# the major third, A the fifth and E the ninth. Said aloud, its notes are D, F sharp, A and E.
String by string, low to high, it plays D, F#, A, D and E. Of the five strings you play, two give D, the note that names the chord. It runs from D on string 5 up to E on string 1.
In Dadd9, only one string rings open, string 1, which gives E. Avoid string 6. Your hand covers four frets, from 2 to 5. There is one more position, starting at fret 2.
Dsus2 is very close. You only move strings 5 and 4. Dadd9 and Bm have D and F# in common. Bm shares two positions with this shape.
Dadd9 shows up in six keys. In D major it is the Iadd9, in G major the Vadd9 and in A major the IVadd9. The rest appear in the key list.