A series of related chords built on notes of a major or minor scale. The chords all share notes from the same scale, and it is this property that allows the formation of a series of chords that will sound cohesive as opposed to somewhat random.
Listen to this song, which uses chords I, IV, and V. Notice that the actual order of chords is I, IV, I, V.
'Lean On Me' by Bill Withers
Listen to this song, which uses chords I, IV, V and VI. Notice that the actual order of chords is I, VI, IV, V - this is a chord progression associated especially with 50s and 60s pop music.
'Stand By Me' by Ben E King
Chords I, IV, V and VI have been used in a vast number of songs. The Australian comedy band 'Axis of Awesome' have taken those chords and shown how many different songs fit the progression I, V, VI, IV.
Axis of Awesome - '4 Chords'
- Chord (N3 & N3 Tech)