If anyone is wondering, this is the C6/9 chord shown on sheet music, a keyboard, and guitar tablature. I have started using the 6/9 chords a lot since they are just so warm and cozy.
A "6/9" chord means that when playing a triad (a chord with 3 notes), you would add the 6th and the 9th note in the scale that the chord is in. So for instance, the key of C has neither sharps nor flats, so C major is [C, D, E, F, G, A, B, C]. To make a C major triad, we take the 1st, 3rd, and 5th note of the series and play them together [C, E, G]. To make this a 6/9 chords, we add the 6th and 9th, so [C, E, G, A, and D].
The sequence in the major scale I provided has only 8 notes in it, so to make a note the 9th, you are just taking the 2nd and popping it up an octave. In this case, D is both the 2nd and 9th, but we say the 9th because we want to have that D on the high end of the note and really sing out!
I mean yea, we can't all be Jacob Collier, but you gotta admit this would sound weird in Pythagorean temperament. Just because you can make it more complex doesn't mean you should.