The round is a common term for a musical arrangement in which the singers divide into two or more groups, and each group sings the same song but begins at a different interval. Thus, at any given point during the round, two or more different verses are heard in unison with each other. It can be confusing and nonsensical if poorly executed. However, in many cases, the confusion of overlapping verses can add a powerful feeling of complexity and significance to lyrics that might seem quite simple, or even trite, when merely read or sung in straightforward arrangement. This effect can be especially strong on the singers themselves, which may explain why this arrangement is often used in liturgical music.
Strictly speaking, the general term for this is a "canon," and the term "round" applies only to cases in which the verses are repeated multiple times (this is also called an "infinite canon"). However, the word "round" is used more often as the general term, especially by non-professional singers.
It is difficult for someone who has not heard music sung in a round to fully understand just how it works. I will try to explain it using an English translation of the traditional French nursery rhyme, Frere Jacques. To keep it simple I will use only two groups, although it is possible to use more.
Often, group 1 and group 2 sing together once through the song:
Are you sleeping?
Are you sleeping?
Brother John?
Brother John?
Morning bells are ringing.
Morning bells are ringing.
Ding ding dong.
Ding ding dong.
Then the round begins; each group sings simultaneously as indicated:
1: Are you sleeping? --- 2: (silent)
1: Are you sleeping? --- 2: (silent)
1: Brother John? --- 2: Are you sleeping?
1: Brother John? --- 2: Are you sleeping?
1: Morning bells are ringing. --- 2: Brother John?
1: Morning bells are ringing. --- 2: Brother John?
1: Ding ding dong. --- 2: Morning bells are ringing.
1: Ding ding dong. --- 2: Morning bells are ringing.
1: (silent) --- 2: Ding ding dong.
1: (silent) --- 2: Ding ding dong.
When three or more groups are used, a short song can continue for quite a long time. Fortunately, it is uncommon for the singers to divide into more than three groups.