it's the phrase/words in a song (like the chorus) that repeats itself throughout the song. ex: in the song happy birthday, 'happy birthday' would be the refrain of that song.
If you type alleluia into a search engine you will find lots of information. The alleluia is part of the Catholic Mass and many other Christian masses or services. Haendel's Alleluia is possibly the...
A chorus is a group song in musical plays /choir while refrain is the repetition of a line at regular intervals at predetermined time both in solo/group song or poetry