seven notes in a major scale=seven chords, built by taking every other note:
in c major
CDEFGAB is the scale
CEG is your first chord, it's c major
DFA is the second, it's D minor
etc...
You use each scale on the chord that's built on it...
Or you can use another scale, depending on the sound you want to get in your improvisation.
Think in modes in terms of "sets of intervals" when you change your mode, the intervals between your notes change...
So, playing the major scale, and making the forth one half step higher makes you play a lydian scale...