When we studied 7th chords we saw that we can move the octave of a major chord back one or two semitones to get a major or a dominant seventh. What would happen if we moved it back another semitone? Would we get a 6th chord? Yes, but In some cases we would get a diminished 7th interval, it depends on the scale that generates the chord.
This particular chord has a minor third, a diminished fifth and a diminished seventh and strangely enough is called a Diminished chord. It's a really peculiar chord because the interval between each note is always a minor third, one and a half semitones and so every note can be considered the root and we get 4 different chords with one position.
Though this chord is really a minor, flat fifth, minor seventh chord (which we didn't study already but is also called a half-diminished chord) where we move back the seventh another semitone, a much more simple way to find it is to take a seventh chord and raise the root by a half step.
So let's take a simple D7 chord and move the root (open D string) up one fret to D# and now we got a D# Diminished chord (or D°).