Chords are the setters of rules within a piece of music. Each moment of any tune is backed by a chord which, for those moments, sets the rules. It determines where the melody can go, what the harmonies will be etc. It is the composer of the song who determines which chords go where, what the progression will be. There are many other familiar progressions which have been the basis of hundreds of hit songs, and there is a whole science to the way chords can lead to each other.
Progressions are often the spark of a composition, rather than the melody.
Chords from within the same key always sound good when played together. Here are some chord progressions that sound good using the open chords. You will begin to recognize chord progressions as you develop your ear for music. They are often referred to by a number system representing each chord's status within its key. We've seen how the scale yields seven chords.