For example, in the scale of C major, the chords are numbered like this: An added 6th is a major triad with the addition of a 6th note above the root. In this example you will hear both the notes ...

As you might suspect, the 6/9 chord contains both the 6th and the 9th of the ‘parent’ scale, the scale the chord is taken from. In practice, the name 6/9 is actually an abbreviation of 6add9 ...