Lunar Threads

What do the elemental icons that sometimes appear in the bottom-right of the battle screen do/mean? I can't remember and none of the guides I've checked mention it.

That's a field effect, which gives a bonus to damage from that element while suppressing opposing element damage. For instance, using Call Earth (a spell unlocked with the Earth Crests) will allow you to set the field effect to earth. I found more detailed information here: http://www.rpgdl.com/forums/index.php?topic=1783.0