Plainchant, or plainsong is a type of early church music consisting of a single line (monophonic) of unaccompanied vocal melody in free rhythm, with no regular bar lengths. It has been present in Christian worship since its earliest days, possibly influenced by Judaism and certainly by the Greek modal system. It was initially the only type of music allowed in the Christian church. It was believed that music should make the listener receptive to spiritual thoughts and reflections, to achieve this the melody was kept pure, repetitive and unaccompanied.
Plainchant is usually either responsorial (where the cantor/soloist sings a series of verses, each one with a response from the congregation) or antiphonal (verses are sung alternately by soloist and choir, or choir and congregation). For now, I suspect most churches will adopt a cantor only approach. A key feature in plain chant is the use of the same melody for various texts. This is similar to ordinary psalms in which the same formula (the "psalm tone") is used for all the verses of a psalm, just as in a hymn or a folk song the same melody is used for the various verses.