The Periodic Condition node adds a periodic boundary condition. This periodicity makes the selected boundaries connected so that they are not walls, and the wall distance values are continuous across the periodic boundaries.

The software automatically identifies the boundaries as either source boundaries or destination boundaries. This works fine for cases like opposing parallel boundaries. In other cases, right-click Periodic Condition and add a Destination Selection subnode to control the destination. By default it contains the selection that COMSOL Multiphysics has identified.