Hey Tim. Good question!
Well, the phrase "7 C major chords" refers to 7 chords of the C major scale
How is that?
In simple words, C major scale is this:C - D - E = F - G - A - B = C
This scale as you can see has 7 notes
If you want to build a chord out of this scale, it's built by taking 1st, 3rd and 5th note
out of the scale.
In the case of C major scale, those notes are C E and G
. This chord (CEG) is know as C major chord
Now, if you create D dorian mode out of C major scale, simply by starting from D and finishing on D:D - E = F - G - A - B = C - D
Now you have built completely different scale using the same notes. The scale is different because the spaces between the notes change place.
Now, let's create a chord out of this scale (mode). 1st note is D, 3rd note is F, and 5th note is A. These three notes played simultaneously create Dm chord
You can repeat this
procedure for every note of the C major scale
So, C major scale has 7 notes
, starting from each note you create 7 scales
, and each scale builds a chord, which creates 7 chords