For example, a racquet could have skips at 7T,9T,7H,9H. This means when stringing the mains, you skip the grommet holes at position 7 and 9 in the throat as well as 7 and 9 in the head. To find your skips, starting from the center grommets, count outward to the seventh and ninth grommet holes on both sides.
https://www.tennis-warehouse.com/learning_center/how_to/string_a_tennis_racquet.html