Please help with A Line dress pattern

I am knitting an A Line dress. It is a free pattern on Ravelry. I am to a point where I don’t understand the directions for the bodice back. It says:
Divide for shoulder extensions as follows: K24 (26)
(30) sts. Place center 24 (28) (28) on a holder. Join
in more yarn and work across the remaining 24 (26)
(30) sts. Working both sides at the same time, work
4 (2) (0) rows in stocking stitch.
Can someone please interpret this for me?! I understand k24, place center 24 on a holder but I don’t understand why do I join in more yarn. Also I stopped after a knit row so If I knit this side it will be one row of garter stitch instead of stockinette stitch. Is that right? I also don’t understand the working both sides at the same time part. Any help would be greatly appreciated!

You need to join in more yarn because your working yarn is on the right needle (you left it there when you moved the middle 24 sts onto a holder) and you need additional yarn to be on the left needle to knit the left side at the same time as the right side for four rows. (you will have working yarn on both sides after that). If you’re only working from one ball of yarn I believe there should be one end coming from the middle and another end from outside the ball.

Thank you for your quick response. I knitted up a swatch of the top part of the dress on scrap yarn so that I could understand what to do. I’m glad I did because then I saw a mistake to avoid on the real dress. I was able to finish the dress and am so happy with it and so is my daughter! Thanks again! What a wonderful site for help!