What is in it
A diminished triad is the root, a minor 3rd, and a diminished 5th. From C that is C, E flat, G flat. Where major and minor stack one large and one small third, the diminished triad stacks two small ones: a minor 3rd (C to E flat) and another minor 3rd (E flat to G flat). The result is that the outer interval, root to 5th, contracts from a perfect 5th to a tritone. That tritone is the whole sound of the chord.
On the staff
Why it cannot rest
Major and minor chords are stable because they are built on a perfect 5th, a wide, consonant frame. The diminished triad gives that up. Its flat 5th makes a tritone with the root, and a tritone is the most unsettled interval in tonal music: it strains to resolve. So a diminished chord never sounds finished. It sounds like a question, which is exactly how composers use it, as the tension just before a resolution rather than the resolution itself.
Where it comes from
The diminished triad is not a special effect; it is built into every key. In a major key, the chord on the 7th scale degree, the leading tone, is diminished (vii°), and it pulls strongly toward the tonic. In a minor key there are two, on the 2nd and 7th degrees. Hearing the chord in isolation is really hearing that leaning-toward-home quality with the destination removed. The tritone in a diminished triad is the same pair of notes carried by the dominant 7th chord of the key: in C, both the vii° (B, D, F) and the G7 hold B and F, which is why the two chords pull toward the tonic the same way.
How Aubel handles it
Diminished is the first chord to unlock after the major and minor pair, the third overall. By the time you reach it your ear already knows the minor 3rd from training minor, so the new thing to catch is the flat 5th and the tritone it makes with the root. The cue is instability: a dark chord that refuses to settle. Augmented unlocks next, the mirror image, where the 5th is stretched instead of shrunk.