"Neck Shaping: Work in patt as set to first marker, join new yarn and BO sts btwn markers, dec as foll: when next st to be worked is a knit st, BO as normal, when next st to be worked is a purl st, k2tog then BO, patt to end, 27 sts rem for each shoulder."
The directions here for how to bind off are for the center sts. You work to the first marker, presumably 27sts, then work the bind off of the center sts. In order to do this, bind off k sts as usual but when you come to purl sts, k2tog, then bind off (This is so you don't have a ruffley neck edge.) Continue to bind off until there are 26sts on the left needle (plus one stitch on the right needle from the bind off) and then finish out the row on those last sts.
When you bind off you knit a st from the left to the right needle. Here they want you to k2tog so that you have one st on the right needle and then bind off.
"Work both sides of neck at the same time with separate balls of yarn. Next row (WS) *work in patt as set across first shoulder, BO 2 sts at beg of opp shoulder, patt to end.* Next row (RS) rep btwn* 25 sts rem for each shoulder. Next row, BO all sts kwise, dec as foll: when next st to be worked is a knit st, BO as normal, when next st to be worked is a purl st, k2tog then BO."
Again the point of the k2tog here is because the k2p2 rib pulls in the knitted fabric. When you bind off either as all knits or even in k2p2 pattern, you'll have too much fabric and it'll ruffle instead of giving you a nice straight shoulder seam. Give it a try and if the shoulder is too narrow you can always take it out and k2tog every other p2 rib.