Since the game's resistence and weakness system is a tad more "clearer" now, I think it's the best to explain it.

Some of your skills will have an icon to it, meaning it's an "elemental" skill.
That means, that this skill is strong against enemies that have this as their weakness.

Here are pictures of what Icon means which element:

So, if for example your skill has the 4th icon to it, that means it will be strong against enemies that are weak to sadism, an element that for example Tarius is good at using. 
The system will be a lot clearer once I've finished the 6th chapter!