It means "dal segno", literally "from the sign". There will be a 'segno' (it looks like a cross between $ and %) earlier in the music, so you jump back to that point and play onwards from there. You might see it as "D.S. al coda" or "D.S. al fine" which means you don't jump back again on the repeat, but instead continue to the end (="al coda/fine"). Addition: Good question Eliot! D.C. is "da capo" = "from the head". So you go back to the very beginning. D.S. is used for going back to a specific point, and is naturally used in longer pieces of music.

D.S? Are you sure it is not D.C?