Stuck in a pattern

I am knitting a sweater, my pattern says to knit across 15 stitches, slip 6 onto a holder which I have done, the next part says to join a second ball of yarn and knit across remaining stitches. Working both sides at once with separate balls of yarn bind off 2 sts at each neck edge.

I am not sure how to join the second ball of yarn and what the rest means.

Can anyone help???

Joining yarn is just leaving a tail and starting knitting with the new strand. To do this part, k15, drop the yarn, put 6 sts on the holder, take the new yarn and knit to the end of the row. ** Next row, work back over the 2nd set of sts, drop the yarn, pick up the yarn on the other side of the held sts, BO 2 and finish the row. Turn and work the first side, drop the yarn, pick up the other yarn, BO 2 and work to the end of the row. Repeat from the ** and go on to whatever it says next.

Awe ok, I think I get it now, thank you so much!!! I am going to go and try now and see how it turns out.