Yes, the person above explained it quite well. You steal the last positive (meaning no weaknesses or anything like that, thankfully) blade that was put on the target. Comes in handy, I'm sure. Ice wizards have one that is like it, except they steal positive shields instead.
I am not sure how much more clearly it can be stated. Your enemy has a charm on them, such as a bonus to damage, a bonus to fire damage, a bonus to accuracy, a bonus to heal amount, etc. They cast it on themselves in a previous round.
Now you come along and cast your Steal a Charm spell on them, and it removes the charm from them and puts it on you instead.
If you still don't understand the the point of it, then don't use it.
If you use this card on an enemy that say has a life blade on it to power its next attack, you will take the blade and add it to your wizard. First the enemy now can not benefit from the blades bonus to its next attack and secondly, if you use a life based attack spell, your spell will be boosted as well.