I’m a fairly new knitter but will help with what I can.
The pattern says to use size 6 double pointed needles. I’ve never done a headband so I’m not sure how/why it is being done in the round. someone else might be more help with this part.
CO means “cast on”. Look up videos on how to do a long tail cast on if you don’t know how.
The I cord is the ties that go under your hair and tie together. Here is a photo from someone else who did that head band which shows the i cord http://www.ravelry.com/projects/lcrooks16/star-stitch-headband
Sl 1 does indeed mean you just move the stitch from the left hand needle to the right hand needle without working it. You slip it purlwise (insert right hand needle into the stitch as if you were going to purl it, but then just slip it off).
Row 8: Sl 1, k1, *YO, k3, pas first of the 3 knit stitches over the 2nd and 3rd stitches; Repeat from * across ending with k1.
My guess is the word pas is a typo and it says pass. So you just knitted three sts, you take the first one you knitted (bottom of the three) and pull it up and over the top of the needle. Check out the video on the bottom of this page, it shows you exactly what to do http://www.theweeklystitch.com/2011/12/knotted-open-work-and-open-star-stitch.html