EXP, or Experience Points, are very important in Metin2.
You gain XP by killing mobs, And Quests.
Depending on the mob's level, you will gain more XP.
If the mob's level is very low compared to yours, you will not gain XP.
If it is very high, you will receive more XP.
Once you accumulate 100% XP, you will lvl up & reach the next lvl.
You might loose a certain amount of XP if you die, killed by a mob (and not a character).