Threading direction is basically which way through a tablet threads are threaded - left-right or right-left. This makes a difference in tablet weaving as the direction of the twining flips based on which direction the threads come out of the tablet.
Tablets can be threaded in one of two directions and are named based on how they appear from above. If the threads go from "top left" to "bottom right" then the card is S-threaded. If from "top right" to "bottom left" it is Z-threaded. The names spur from the direction of the diagonal in the letter iself (S or Z).