When you turn your work you literally turn it around and start working on the other side. The RS is side you want to show to the public and the WS is the back side. Obviously with a scarf both will show, but you need to keep track so you knit the pattern correctly. You can put a locking marker or safety pin on the front of the work, the RS, to help you keep track at first. After awhile it's pretty obvious which side is which with most patterns unless they are reversible.
That's a slip stitch pattern. You slip the stitches purlwise..which is when you stick your right needle into the stitches from right to left and slip them from the left needle to the right needle. Like when you purl, but you won't be working the stitches. You also have to have your yarn on the WS as you slip so watch that, too.