PinWheel Sweater Help!

Ok Soooo I’m alittle confused…
I’m at the part where you put stitches asside for the sleeves.
pattern :
place the second set of 30 stitches on a holder, join a new ball of yarn and cast on 30 stitches

ok … soooooo i get put 30 stitches on holder… but what do they mean by join new ball of yarn… where did the other ball of yarn go ??? ahh confused…
please help

You probably can cut the yarn from where it is now and use that. In other words, you won’t be using the yarn from where it is anymore, but from where you cast on the new stitches.

Or you can leave the other ball of yarn there, and use it on the sleeves.