It's to reinforce the heel flap, you can choose to do a regular stockinette stitch like the rest of the sock. Also, fyi, I have found that when doing a heel flap that the slip 1, knit 1 on 1 row and slip 1 and purl the remaining sts and cont. this 2 row repeat on the heel is enough reinforcement....IMHO. When I have done the sl 1, k1 on 1 row, then the sl 1, p1 across the row and continue it this pattern that it becomes too tight of a heel; but you do have a different look ... just my preference. Any help at all?!