Dal segno is a musical instruction found in sheet music. It tells a musician or singer to go back to a particular place in the music and repeat it from that point.
Like many musical instructions, dal segno is Italian; it means "from the sign." The sign, or symbol, to which the musician is to return is referred to as the segno, which looks like a fancy S with a slash through it and a dot on each side. The dal segno instruction is often abbreviated as D.S. in sheet music, and it's often followed by other instructions: D.S. al fine, for example, means "from the sign to the end."