Homophony means music in which the voices or instruments sing or play chords (chords are when two or more notes are played together.) In homophonic music all the choir (sopranos, altos, tenors and basses) are singing the same words at the same time. There is a tune on top and the lower parts are the accompaniment. This is what happens in hymn singing. The opposite is polyphony. Polyphonic writing is more complicated: the choir sing different melodic lines at the same time (see counterpoint). The terms "homophony" (literally: "one sound") and "polyphony" (literally: "many sounds") are mostly used for choir music.
In homophonic music it is easy to hear the words that are being sung. In polyphonic music it is much harder for the listener to understand the words. Composers often used polyphony when writing music for the mass because everyone knew the words anyway. Homophonic music became more important when composers started to write operas and madrigals where a story is being told and the words must be heard clearly.
Homophony in Tallis' "If ye love me", composed in 1549. The soprano sings the melody (the primary line) while the lower voices fill out the harmony (as supporting lines). The rhythmic unison in all the parts makes this passage an example of homorhythm. This example can be heard in the listening sample below.
Homophony for Kids. Kiddle Encyclopedia.