sl1 / slip 1 leaves a stitch unworked, but moved to the right needle from the left.
On RS rows: With the yarn in back as to knit, move the first stitch in the row to the right needle, with the right needle going into the stitch as to purl, and without working the stitch
On WS rows: With the yarn in front as to purl, move the first stitch in the row to the right needle, with the right needle going into the stitch as to purl, and without working the stitch.
