This item gives a x1.5 xp bonus in one of five available skills up to a total of 34,400 XP. The available skills are determined as being the five lowest skills that you have (determined by XP, not level). Note that a Dungeoneering level of 120 counted as a level 99 for this pendant.
To activate the bonus, right-click on the item and choose 'Gain xp'. You'll be asked which skill you'd like the bonus to go in. Choose your preferred skill and the bonus XP provided by this item will be added to your bonus XP pool.
Once the xp boost has been activated, the pendant will become cosmetic.