Hope this will help:
When it says “finish off the top”, it should end with a colon: it’s about to give you the instructions for finishing off the top!
It’s a common cinch top. All of your stitches are still on the needle, cut a long tail length of yarn and thread that strand onto a yarn needle (for sewing). Pass the yarn-threaded needle through each stitch on your knitting needle (you can even drop the stitch from the knitting needle as you thread your sewing needle through it). Once your sewing needle is threaded through all of the knitting needle stitches, remove all of the knitting needle stitches from the knitting needle and pull your yarn-threaded sewing needle - it will cinch up your piece to a closed top! If you loop the sewing needle through a stitch on the far end, it will create a closed circle.
So you can sew the side seam without the top of the hat being too straight!
The “rib section” is the bottom couple of inches when you were doing a K2/P2 pattern, creating a “rib” design like that used at cuffs and collars. Since this edge will be partly folded up on the finished hat, the pattern is suggesting you switch your finishing side seam to the outside for the final bit, so that when you fold it up then the seam won’t show.
how does the hat look, in the end? Good luck!