Diatonic modes are the scales that are formed in a major key when using a different scale note as a starting and stopping point. We have been starting and ending on a C note. We could do the same with an E note, but still using the notes in C major, and get an entirely different sound. Since there are 7 notes in a scale we end up with seven modes. Matching the mode to the Chord based on that note can be great when finding new improv lines, and once again, will give you a whole new perspective on C major and the possibilities of the scale.