The diatonic scale is a starting place for available chords to choose from in a given key, in your score. Like a roadmap the scale provides direction or information on where to go on your musical journey. These chords are triads built on the root of each degree. Each scale degree has a name and number. The first scale degree is called the Tonic or the I chord. There are major and minor chords within the diatonic scale- designated in upper and lower case.
The chord mapping tool illustrates the progression of chords going from left to right beginning with the I chord. You may skip chords going forward and can come back to certain chords as shown by the arrows. After leaving the tonic the point is to eventually get back to the tonic or ‘chord of repose’.