Knitting baby sweater: what is "knit both sides at once"?

Knitting my first baby sweater, a pullover, and reached the neck. Pattern says: "join second ball of yarn, and knit both sides at once"
What does that mean? Help!

It means that you are going to be knitting the left and right sides of the sweater at the same time but with 2 separate balls of yarn. You will knit across with the first ball of yarn and at the neck you will either bind off or put the stitches on a holder. Then you join a second ball of yarn and continue across the row to make the other side of the sweater. When you turn and work the next row you continue with the second ball of yarn until you reach the neckline. Then you drop that yarn and knit the other side with the first ball of yarn.

You will continue to knit the shoulders of the sweater by alternating the balls of yarn until you finish the shoulders and bind off.

So you’ve just bound off some stitches to make the neck opening. Now, you have a set of stitches on the right hand needle, then the opening. You’ll join a new ball of yarn, and knit the rest of the stitches left on the left hand needle. Then turn the work, knit with the new ball again, then you’ll come the neck opening, drop that ball of yarn, grab the ball you had been using, and knit across that side of the sweater. That is knitting both sides of the sweater at once,