In music notation, dal segno is abbreviated "D.S." and is positioned above the staff in the same manner as a da capo marking. It indicates that the music repeats from the segno (sign), and is usually used to repeat passages that make up a very large part of the musical piece. The phrase simply means "from the sign" in Italian.
When in conjunction with the segno on the staff, they look something like this (a dollar sign ($) is the closest ASCII approximation to a proper segno):
/\ $ D.S.
---| /----------------------------|---------------------||
|/ | | ||
---/------------------------------|---|----|------------||
/| 4 | | | | * ||
-/-|/\--------------------|----|--|---|----|---*---|----||
| | | 4 | | | | | * | | ||
|--|--|--------|-----|----|----|--|--*--------|----|----||
\ | | | | | * | | | ||
--\|_/---------|-----|---*--------|-----------|---------||
| | *
\| -*--