it's just like you see in the image.
You always consider 100% as normal. Soo if something goes above 100% is a boost in your damage if you use that element, if goes below means that you wont be using that element because you'll do less dmg..
Monsters element goes from lvl 1 to lvl 4, wich means every level indicates how much stronger against one element and weaker on another element the monster is.
Using that Ifrit example:
Neutral 100% - means you dont get any bonus dmg using neutral element on him
Fire -50% - your fire atk will completely miss
Earth -60% - you'll have 40% less dmg using earth element on him
Wind 100% - same as neutral, soo you dont get any bonus using wind
Water 200% - you dmg will double using water element
Poison/HolyShadow 75% - same as earth elemente but instead you'll be losing 25% damage using this element
Ghost/Undead 25% - same as above but losing 75% dmg using this element
Hes is a lvl 4 monster soo you see he's very weak to water, but immune to fire.