Chord Progressions dewd. Just look at any piece of sheet music you may have, and you'll see stuff like "Gmaj7" above the staff. That's the chord progression of a song. Songwritters use those as building blocks for writting a song. They are the backbone of every song. Melody and harmony are all build from the chord progression of a song. Understanding them and how to apply them is alot deeper. I suggest looking into music theory studies if you want to know more.