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.