So, as mentioned in the introduction, 5 (power) chords become drastically simpler when tuned to drop D. If you think about it, a power chord in standard tuning contains two notes; any note on the low E string plus its fifth, which is usually down a string and up two frets. Now that our low E string is tuned down a full step, we compensate by bringing that index fingered note up two frets. Now, we play the fifth and root by simply barring the top two strings. By including the D string as well, we get a full power chord complete with root, fifth and root octave. This is what Exercise 1 demonstrates.
In Exercise 2, we take a look at the seven chords diatonic to the key of D minor, harmonized in thirds. Here we spread the fingers out a little and enrich our power chords with some third harmony. These chords are frequently used by those who prefer drop tunings as they occupy a lot of "space" while not cluttering up the rhythm section with extra octaves - we're sticking to the root, third and fifth.
Exercise 3 offers a cool way to tie some of these chord ideas together in a progression. Notice the third which transitions to the second at the tail end of each chord in the progression, experiment with some of your own chord voicings in much the same way!