Nehtdx

The seventh chord built on the fifth step of the scale (the dominant seventh) is the only dominant seventh chord available in the major scale: it contains all three notes of the diminished triad of the seventh and is frequently used as a stronger substitute for it.

What does this mean?
Especially "diminished triad of the seventh"?

"Dominant seventh" is a shorthand for what others call a "major-minor seventh," meaning a major triad with a minor seventh on top.

If we take all notes of a major scale and create seventh chords on top of them using only the notes of that major scale, only one of these seventh chords will be a major-minor ("dominant") seventh: that built on scale-degree 5 of the major scale. Scale-degree 5 is called the "dominant," hence the term "dominant seventh."

When this text says "diminished triad of the seventh," the writer is being a little loose with their terminology. It should say "diminished triad of the seventh scale degree," because the triad built on top of that seventh scale degree is a diminished triad. And that same diminished triad is the third, fifth, and seventh of the dominant seventh chord built on scale-degree 5.

In C major, scale-degree 5 is G and scale-degree 7 is B. The seventh chord on G is G B D F (a major triad G B D with a minor seventh G F), and it includes the diminished triad built on scale-degree 7: B D F.

We call the resolution stronger because the V7 chord is much more directional (the vii°7 chord, which would be the diminished seventh chord, is symmetrical and ambiguous). There are other reasons, and as further reading, this post does a good job explaining this resolution's strengths.