Not a new player, but kind of a noob question, heh

[Lethallin]

That's 50% if you have any higher level battle class. It doesn't have to be at 50, just at least one level higher than the class you're currently leveling.

It's actually +50% up until the exact point that your highest level class is. If you have a level 40 with 10k XP, you'll get the bonus xp until exactly you hit level 40 with 10k xp with a different class.