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, 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.

slip 1 - insert needle into stitch as normal then just pull it off the left needle onto the right without knitting or purling it

Inserting the right needle into the stitch on the left one, and transferring the stitch from the left needle to the right one.

Insert the knitting needle into the loop as if to knit (as opposed to the direction you insert to purl), slip the loop onto the inserted needle without creating a new stitch.