What is a Diminished chord? A diminished chord is root + 3 semitones up + another 3 semitones up + another 3 semitones up .. and another 3 semitones up brings you back to the root.

Here is another fascinating video about how and when you can/should use a diminished chord.

5 Easy Ways to Use and Write with Diminished Chords [MUSIC THEORY - CHORD PROGRESSIONS]