I won't explain much of chord theory since it seems clear that you are looking into theory on your own. however, I will explain what is written in the snippet that you posted in your question.
"The seventh chord built on the fifth step of the scale (the dominant seventh)"
it is referring to the diatonic seventh chord derived from the 5th note of your major scale.
Diatonic seventh chord means the chord has the triad plus the seventh note which is in the scale.
For example - in the case of C major scale, the fifth of C is G. Now let's try forming the seventh chord (remember the seventh chord is basically the triad plus the 7th note from the initial note.).
G triad in C major is a major chord - G-B-D.
the diatonic ( in scale ) seventh from G is F.
So your seventh chord built on the fifth step of C major is G-B-D-F.
This chord is known as a dominant seventh chord and is denoted as (roman numeral)7.
"is the only dominant seventh available on the major scale"
There are 4 types of 7th chords. ( you can look it up on youtube as I don't want to get into too much theory here )
and what the above line meant was that you can only make a dominant 7th chord from the 5th note of the major scale within the scale. Taking any other note and making the seventh chord from that note won't be a Dominant seventh.
"it contains all three notes of the diminished triad of the seventh and is frequently used as a stronger substitute for it"
the diminished triad of the seventh means the chord root of which is the seventh note of the scale. So in the case of C major scale, it will be a B diminished chord.
The notes of B dim are - B,D,F
It has almost all the notes from G7 or "the seventh chord made from fifth step".
Since it has almost all the notes from Bdim, it is often times used as a substitute of the diminished chord.
I hope I was able to give you a satisfactory answer. If you have any more queries regarding music theory, feel free to ask :)