Pop songs are always in a ‘key’. This means they use certain chords that go together well, which are derived from corresponding the major scale. Music teachers always explain the key of C first as it has no nasty ‘sharps’ of ‘flats’.
These chords can be seen like colours that go together well- meaning any of these are a safe choice and a song in the key of C would mainly use these chords and few others, if any. As well as the chords, we can play notes from the C major scale (next page) over the top of these for the melody
Example chord sequences in the ‘Key Of C’
N.B. The info on cadences above differs somewhat to their dictionary definitions. This was chosen to communicate the heading of the key of a song to beginners, while being helpful at understanding keys. More to come on cadences in the future!
For further reading on this topic, you can learn about the key of G in this video here.