The leader (the dancer in front) turns away from the center of the box and walks back to where the other dancer was standing, turning all the way around (360 degrees), to stand in that spot facing the same direction again. Meanwhile the trailer (the dancer standing behind) simply walks forward into the front spot.
Usually there will be two boxes doing this at the same time. The dancers in each pair stay in that pair, and the dancers in each box stay in that box. Nobody crosses the center of the overall formation into the other box.
Note that from a formation where one couple is standing in front of another couple, as the leaders turn away from the center of their box one will be turning left and one will be turning right.
The result in this case is that the couples have switched places, with the couple that had been in front now standing behind the couple that had been in back.