Baby talk!

[Lin]

HI, Could someone help me by explaining these instructions in words that a baby could understand??
"When piece measures 19 1/2", shape neck: Work 30 sts, join second ball of yarn and b.o. center 37 sts, work to end."
I'm a bit confused as to how this is all going to work out....Will I use another needle?? What is going to happen to the second ball of yarn after I have bound off the stitches?? Please, anyone who is explaining....simple and detailed...LOL...Thanks so much! Linda