Okay, you've knit 6 rows of garter - k,p,k,p,k,p - which leaves 3 purl ridges on each side. The next row you start the 6 rows of stockinette - k,k,k,k,k,k. Turn the piece to the back side and there will be 6 purl rows uninterrupted by a knit row, like the garter is. That's the 6 rows of stockinette. Turn back to the front side and continue with the 6 garter rows.
Does that help?