XP stands for experience points, there are many ways of getting this, you can kill things, starting from orphans usually and moving up. Or you can craft things, gaining a specific amount of xp after each successful craft, or you can find and complete journals, which give a specific total of experience to the players character. Experience points are used to increase skills, to know how much you can use on skill check the number beside Unspent XP in your score screen. The number of Total xp is just that, your total number of xp earned, whether through journals, crafting, or killing things. Your effective xp is the amount of xp you've earned from either killing or crafting, meaning your character was actually learning something in the process. Lastly you can also get small amounts of XP from admins as gifts for doing something they ask, or if they deem you worthy of the gift.
You lose XP when either you train, giving up the amount needed to train a skill or stat up to the next level, or you die, losing a small amount of unspent xp. Without enough xp you will not be able to increase your skills and will be stuck until you gain some. Remember, after you reach a certain point, lower "level" mobs, such as orphans, will stop giving you experience, forcing you to fight things more along your level.