From FlyFF WIki
When a player delevels, this means they had less than 4% in their exp bar and died. Instead of reducing the exp down to 0%, 3-4% of your exp will be deducted.
If you are wearing an Armor set or a weapon that meets the level requirements of your previous level and higher, you will no longer be qualified to equip them. Usually if you die with these items already equipped, you may continue to use them until you unequip them, in which you will not be able equip it again until you meet the requirements.
A level 60 Psykeeper has died and lost experience, resulting in a delevel to 59. It just so happens she was wearing her level 60 set and wand. She can continue to use these, however if she unequipped them after she had deleveled, she would no longer be qualified to wear them until she levels to the required level ('60') again.
What about my STAT and Skill points?
With Skill points, any new skills you have obtained will be unuseable if you delevel past the required level for that skill. Your skill points, however, will not be returned for redistribution and the skill will still be the same level once you reach the required level again.
Stat points are redistributed the same way.
Resurrections and Blessings
Assistants who have a max (level 20) Resurrection skill can decrease the amount of experience that is deducted from your character. The experience that is saved when you are resurrected changes according to the level of the skill.
Using a scroll is extremely convenient for those who have no supporters, and unlike the Assist's Resurrection skill, these scrolls prevent all exp loss, not just a fraction.
When leveling a peer, make sure to give them an extra 4-5% as insurance to their newly regained level.
Experience Boxes were introduced into the game in v7 and used to hold part of the experience lost by character upon its death. If the character returned to the place of its death before the timer on the Experience Box ran out, they were able to open the box and get a portion of the exp lost back. Experience Boxes were removed from the game with the upgrade to v9.