This is intended for members of our choir who are not familiar with some of the indications in the written music that we use.
Written music has a few symbols to indicate that sections of the music should be repeated.
So, this section of music means that the 3 bars between the left repeat-sign and the right repeat-sign should by played or sung twice.
If I use the letters A, B and C as the names of these bars instead of showing the notes in them,
then the section above (with repeats) is a shorter way of writing it than as below (without repeats).
Obviously, if the repeated section has 20 bars instead of just 3, this can save a lot of writing.
On the 2nd pass through, you should jump past the 1st time section, and continue at the section marked like this.
So, this section of music
is a short way of writing bars ABC, then DE, then ABC again and then bars FG.