High tide and low tide has to do with waters literally, but I think metaphorically it could be about emotion. A high tide is when the waters are in on the shore. This could be a metaphor for when you have lots of energy in your life and everything is going well for you. A low tide is when the waters are out. When it is low tide you can usually see far more of the beach and the sand that is normally covered with water is laid bare. This could be a metaphor when things are not going well in your life.