I, IV, and V (and their variants, i, iv, and v) are the
chords that form the backbone of
music in
Western culture. The roman numerals represent the relationship of the chord to the
tonic. Capital letters represent
major chords, lower-case represent
minor.
For example, in the key of C, the I, IV, and V chords would be C, F, and G, respectively. In A minor, the i, iv, and V would be Am, Dm, and E respectively.
In harmonically simple styles of music (folk, rock, children's songs, etc.), the majority of the songs are written with just these chords.