Each of your units and those of the enemies have elemental affinitiess that are weak and strong against each other. This becomes particularly important in boss battles where you will do more damage if an element is weak against yours and half if the elements are the same. Listed below are the the elemental match-ups.
Fire and Ice are strong against one another
Lightning and Darkness are strong against one another
Solar and Lunar are strong against one another
Photon and Graviton are strong against one another
In addition:
Solar is strong against Ice
Lunar is strong against Fire
Photon is strong against Darkness
Graviton is strong against Lightning