A character must be on or adjacent to a node in order to activate it. To activate nodes, you must use spheres. Spheres and sphere levels are gained by getting ability points which are obtained by participating in battles. If you want to activate nodes you passed over, you must spend sphere levels to go back. When you're retracing your steps, you can move 4 steps by spending 1 sphere level. However, backtracking is not very cost-efficient, so plan ahead to avoid wasting sphere levels.