This exercise is from the Theory section on Keys.

Here we are playing in the key of C major. Even though we have different chords playing in the background, C is the dominant tone - the key note. Moreover, the whole C major scale will work over this entire chord sequence.

First we play out the backing chords by themselves:

C major - F major - G major - C major

Then those chords repeat in the background whilst we play out the C major scale notes over the top. We vary the scale a little towards the end to make things a little more musical.

The point of this exercise is to see that when playing in a Key, everything else is relative to the dominant key note - and its corresponding major scale!

The C major scale is unique in that it contains no #'s or b's. The notes are:

C - D - E - F - G - A - B - C

The chords in this ActionTab come from this 'harmonized C major scale', giving us:

I C major
ii D minor
iii E minor
IV F major
V G major
vi A minor
vii° B dim
I C major

So, we are just playing a chord progression of I - IV - V - I in the key of C major. See the section on the harmonized scale if you are lost!