Repeat marks of various type indicate to the singer that a section of music should be repeated:
- Two dots (or sometimes four) to the left of a bar line like this:
indicates that you should repeat from a similar mark where the dots are on the right of a bar line like this: 
or there isn't one, the start of the piece. So the section between the two bar lines with dots, or from the beginning to the set of dots is sung twice.
- Da Capo or D.C. means repeat from the beginning, so the section from the beginning to the D.C. mark is sung twice.
- Dal Segno or D.S. means repeat from the sign, and the sign is usually a symbol like this:
above a previous bar line. So the section from the sign to the D.S. is sung twice.
- Repeat marks can also be used with 1st and 2nd time bars. At the end of a repeated section, one (or sometimes more than one) bar is marked with a line aross the top with a small "1" or "1st" indicating that this bar should be sung the first time. This will immediately be followed by a "second time bar". This is easist to show in a simplistic example:
The bars are sung in this order: 1, 2, 1, 3, 4
