Added tone chords are major triads with an extra note added. They are abbreviated "add" and there are mainly two types: add9 and add2 (there are also the less common add11 and add4). In the add9 the ninth tone in the scale is added and for the add2 the second tone in the scale is added.
See diagrams of add chords*:
C add D add E add F add G add A add B add
Cm add Dm add Em add Fm add Gm add Am add Bm add
C add11 D add11 E add11 F add11 G add11 A add11 B add11
* To avoid too high information density, only naturals are listed. You can easily reach, for example C#add or Dbadd, by first click on "C add" and when click on to the next chord.