Killing enemies yields experience (Exp .) When you aquire enough Exp your character will level up, giving you 50 more HP and 10 more AP. If you level up enough you can also equip better armor and weapons. lower level creatures will give less exp than higher level ones but the higher level ones are harder to defeat so if your trying to level up go to an area with creatures around your level i'd reconmend evermore greens for levels 1-5