sl1-It means to slip 1 stitch from the left needle onto the right needle with out knitting it.
k2tog- it means to knit two stitches together as 1 stitch.
psso- it means pass the slipped stitch over the stich you mader after knitting the two stitches together as 1 stitch
it is a type of double decrease
hope this helps.
Daylilies are the Lord's smiles, a new one every day.