It just means turn it around. You're working in a tube going down the leg of the sock. When you get to the heel flap, you're going to ignore some of the stitches and work the others like a piece of flat knitting. So you'll knit to the end of the row, and then turn and knit the next row just like a piece of flat knitting.
Amy has a great tutorial on sock making, that I encourage you to watch.
Scroll down on this page to see "Working the heel of a sock."