Just like the heel flap, you're working it flat instead of in the round. Normally with a flat piece of knitting, you turn the work at the end of each row (so you can go back in the other direction). When it says turn work, you turn it as you normally would at the end of each row, except you're turning before you reach the end.
So work as many stitches as it says (it won't be all the way across) and when it says turn, turn the work over and start working back in the other direction.
Here, Silver does a superb job of explaining it: http://www.cometosilver.com/socks/SockClass_Heel2.htm