Crochet Help: what does From RS, join yarn with sl st in 7th (8th, 9th) st. mean?

I’m making driving gloves (pattern: http://www.lionbrand.com/patterns/70713AD.html?noImages= ) and i just don’t get this part
I do know how to slip stitch I just don’t get :join yarn with sl st in 7th (8th, 9th). I have no problems with the abbreviations.

You started with the wrist and created a flat part that’s fastened with a button through a buttonhole. Now you are going to start the hand part, which forms a tube, part of it attached to the wrist part and part of it unattached but covering the back of the hand.

Start at the “start” point and count 7 stitches in for small, 8 stitches in for medium, and 9 stitches in for large. With a slipknot on your hook, make a slip stitch in the 7th, 8th, or 9th stitch (depending on the size). Then start single crocheting around the next 6, 7, or 8 stitches (depending on the size). Then you skip 8, 10, or 12 stitches to make the opening for the thumb. You might want to refer to the picture, find the thumb opening, and look at where the back of the hand joins the side of the thumb. That’s where you started crocheting this round.

Crochet Fingerless Gloves