It means you are adding some stitches for the sleeves at each side of the work, so that would make 11 hdc (counting the turning chain) on one side, hdc each hdc in the row below across work, then chain 13 and repeat going back the other way. At the end of that second "ch13 1hdc in 3rd ch from hook. 1 hdc in next 10ch. 1 hdc in each hdc of row" you will have 58 hdc upon which to work your next row.
I don't know the pattern, but I'm assuming it is worked in one piece, bottom front to top to bottom back, with added stitches at the arms to make sleeves. Then you simply seam the sides up and under the sleeves. Super slick.