If XP is anything like VX, (which I assume it is), The elements efficiency rating has to do with how strong a monster is against that element. C is normal damage, with B and A meaning the monster takes less and less. D, E and F mean more damage taken from that source or more likely to suffer that state. With weapons, the + means that the weapon has a chance to inflict that state on the enemy struck. Same thing with elements, if + the weapon has that attribute.
VX has this feature, try it in XP: right click on things you don't understand, then left click on the "what's this?" box. It's a great tool.