According to the official website,
Level Sync is a party leader-activated system whereby the level of all party members is restricted to that of a designated player. When this feature is activated, all members of the party will be able to receive an amount of experience points corresponding to the level of the designee, regardless of any initial disparity between them.
http://www.playonline.com/pcd/verup/ff11us/detail/3668/detail.html
If you have missed FFXI this is your chance to join one of the largest digital worlds out there, as it is now more newbie friendly.