Hey Friend,
Moving right along, let's start looking at the next chord in the key of C major - E minor.
If you recall, this chord is the "IIIm" or the three chord from the harmonic progression in the key of C major. Remember...
C, Dm, Em, F, G, Am, Bdim
Construction of the chord comes from looking at the key of E, where we have notes E, F#, G#, A, B, C#, D# and back to E.
Taking the normal formula of 1 3 5 for the notes in the build, we get E, G# and B, but using those notes would give us the E major chord,. Sooo, we need to flatten the 3 note (move it down by one fret or half step), which give us the minor chord. The notes are now E, G and B.
Like we say with the Dm chord, the notes for E minor are already in the C major scale. Here's how they line up:
C D E F G A B C
I don't know about you, but the mechanics of chord construction are fascinating to me. I mean, even though a chord is based on a specific key (E minor is based on E), you still have the notes for Em in the key of C. It took a few times looking at this for the light bulb to finally come on in my head with this stuff, so don't worry if it's still fuzzy for you. We'll come back to it time and again.
This is a very easy chord to play in the first position. It is a second inversion chord, so the note order when you play it is 5 1 3, or B E G.
You'll start with an E shape chord with middle finger on the fifth string, second fret, ring finger on the fourth string second fret and index finger on third string first fret. This gives you an E major chord. We already know that to make a minor chord, we need to drop the 3 note down by one half step (one fret) to get our minor third interval.
With the E chord, that simply means lifting you index finger off the third string an playing it open. The tab exercise plays on that with you alternating between E and Em chords.
E minor is a very useful chord. It's not only found in the C major key, but also D major and G major
Key I IIm IIIm IV V VIm VII° C C Dm Em F G Am B° D D Em F#m G A Bm C#° G G Am Bm C D Em F#°
And of course, the minor keys of A minor, B minor and E minor:
Key Im II° III IVm Vm VI VII Am Am B° C Dm Em F G Bm Bm C#° D Em F#m G A Em Em F#° G Am Bm C D
That should keep you busy for today. See you tomorrow!
Peace~
Dave | 
