Heel flaps are made denser than standard stockinette because they get alot of wear along the back to the shoe.
One way to do this is a slip stitch pattern. You slip every other stitch starting with the first one. It must be worked on an even number of stitches. The slipped stitches can be either on the knit side or the purl side. Just as long as you are consistent.
This pattern calls for slipping on the RS (knit side.) You slip first stitch purlwise, knit 1, slip 1 stitch purlwise, knit 1, repeat across the whole row. Flip to WS. Row2 (WS) Slip first stitch (you slip the first stitch of all rows to make a nice edge to pick up stitches later.) Then purl across all the stitches on that row. Row3 (RS) you repeat row 1.
To slip purlwise, you put the needle into the stitch in the same manner as if making a purl stitch. You just don’t wrap the yarn around. The stitch passes unworked from one needles to the other. And the yarn stays in the back, to work the next knit stitch.