There is a line in the first verse of Leonard Cohen's Hallelujah -
"Well it goes like this: The fourth, the fifth, the minor fall and the major lift"
All musicians (and many people) will know that what he is describing here are the actual chords being played in the song at that time.
Most popular music (folk, country, rock) are based around 3 chords that naturally "sound good together" to the ear - the root (or first) chord, paired with the fourth chord and the fifth chord.
To give a simple explanation, think of the root chord being the thumb of your hand, with the fourth and fifth being the ring finger and pinky finger respectively. So, if you are playing a song on an instrument, you can start on any chord and find it's complimentary chords by looking at your fingers,
For example..
Disregarding sharps and flats, there are 7 major chords - A B C D E F and G
So the complementary chords would look like this...
1st 4th 5th
A D E
C F G
D G A
E A B
G C D
and so on...
Understanding this musical relationship, you can play any simple folk song, starting in any key, as long as you adhere to the above formula. It becomes helpful if the song you are playing is out of your vocal range, and by changing the key up or down, the song will be easier to sing and sound essentially the same.
The reason I'm bringing this up is that some musicians think that by making a song more complex, adding a bunch of different chords (diminished 7ths or augmented 9ths - jazz territory), outside the natural 3 chord complementary relationship, automatically makes the song better or more interesting somehow.
But for me, a songwriter who can make a song interesting and dynamic using only 2 chords, is something very intriguing and worthwhile, not to mention quite difficult to achieve. One of the most famous examples I think is "A Horse With No Name" by the band America. Personally, I find this song rather tedious after a while, and becomes a little grating on my ears. Still, it's popular and very well known.
I was listening to a couple songs the other day which I think are good examples of the songwriter's ability to make a simple song sound engaging and varied, even though they essentially use 2 chords - the root and the 4th.
The first one is by Lucinda Williams...
The next one by Bob Schneider...
Now with this in mind, listen closely to the following track by Brett Dennan. It's a lovely song, where he also is mostly relying the first and fourth chords, but adds the complementary fifth chord in only 2 places. See if you can hear when it happens...