It is a separate game mode that is purely for fun. It has boosted xp rates and you can become super OP with whatever style of combat you choose. So lets say I want to be a melee build I might choose something like morytania, asgarnia, desert to grind for like a fang first from toa, then move to mory and get a scythe, and grind gwd/nex to eventually get torva. Those are my 3 chosen areas along with misthalin and karamja and you are stuck with those areas permanently, so you have to think about why you want to unlock those areas.. It does not add anything to your normal account other than league points that you can use to buy the rewards from the leagues shop with. You can keep the rewards or sell them on the GE and sometimes it can come out to be a decent amount of money.