In knitting, sl1 means to slip one stitch. To do this simply slip the stitch from the left hand needle to the right without knitting it. Stitches can either be slipped purlwise or knitwise. If slipped purlwise, the stitch will not twist but if slipped knitwise it will twist. Therefore, if the pattern does not specify which way to slip the stitch, it is best to slip purlwise. See the related link for a diagram showing how to slip stitches.
SKPO stands for slip knit pass over. In other words, you slip one stitch, knit one stitch, then pass the slip stitch over the knitted stitch, thus decreasing one stitch in your row of knitting.
In knitting, SSK is the abbreviation for Slip, Slip, Knit, and is one of the most common ways of decreasing on a left slanting decrease.TO SSK (For right-handed knitting):Slip one stitch from left-hand needle to right-hand needle as if to knit. (Also said slip one knitwise).Slip the next stitch from left-hand needle to right-hand needle as if to knit.Insert the left-hand needle down into the front of the two stitches just slipped, and knit them together.