Ruins indicate the general rise and fall of cultures as time passes in human history. They are about the transitory nature of time, they are a connection the past, grounding because of our connection to a greater thing than ourselves, yet unnerving because they tell us about our mortality, and the passing of cultures and times.
They can also sometimes be cautionary, saying, "Look, these people had advanced buildings, technology, society... where are they now? What happened? What was their weakness and downfall? How can we learn to avoid those weaknesses in our society? What do we do now that is not in the interest of our own survival?"
See the other entries in this series about symbolism of: The Undead, Deserts.