What is in it
An augmented triad is the root, a major 3rd, and an augmented 5th. From C that is C, E, G sharp. It stacks two major 3rds (C to E, then E to G sharp), the mirror of the diminished chord's two minor 3rds. Stacking two large thirds pushes the outer interval past a perfect 5th into an augmented 5th, a semitone wider than stable.
On the staff
Suspended, not pulling
The augmented chord is unstable like the diminished, but it feels different. A diminished chord pulls sharply toward a resolution; an augmented chord floats. The reason is symmetry. Three major 3rds split the octave into three equal pieces, so C augmented, E augmented, and G sharp augmented are all the same three notes. No note sits in the chord as an obvious root, and with no root there is no clear direction home. The sound hangs, which is why film and theatre writing reaches for it to colour a moment of suspension or unreality.
Where it comes from
Unlike the other three triads, the augmented chord does not occur on its own in the plain major scale. It arrives through chromatic alteration, when a composer raises the 5th of a major chord, and it lives natively in the whole-tone scale, whose six evenly-spaced notes contain nothing but augmented relationships. That even spacing is the source of both its symmetry and its dreamlike, placeless quality.
Jazz makes that relationship explicit. Mark Levine treats the whole-tone scale as the chord-scale for a dominant 7♯5 chord, with the augmented triad sitting inside it, and notes that because the scale is built entirely of whole steps there are only two of them in all of music. It is a colour used in short doses.
How Aubel handles it
Augmented unlocks fourth, the last triad before the 7th chords begin. It is the counterpart to the diminished chord you trained just before it: where diminished shrinks the 5th, augmented stretches it. The cue is that even, suspended stack of two major 3rds, bright on the bottom but with a top that will not settle. After it, the dominant 7th opens the run of four-note chords.