A sequence of Chords giving movement to Music. Usually, but not always starting and ending on the Tonic or root note of the Key.
Commonly written using a notation based on roman numerals. In upper case meaning Major and in lower case meaning Minor. In the key of G for example, G major is I, and E minor is vi.
Common chord progressions
I-IV-V or I-IV-V7
The 3-chord trick
The Fifties songs