sl = slip

Sometimes stitches are slipped without working from the right needle to the left needle. They can either be slipped purlwise or knitwise. If you are asked to slip stiches and the instructions don't specify which way, slip the stitch or stitches purlwise. 

Slipping Stitches Purlwise 

Insert the right needle (from back to front) into the next stitch on the left needle and place it on the right needle without working it. 

Slipping Stitches Knitwise 

Insert the right needle into the next stitch on the left needle as shown and place it on the right needle without working it.