You'll understand atma after (and if) you do your relic quest. It's basically just a randomised drop from FATEs you need to gather to upgrade your weapon, but the nature of truly random drops is that they're massively unfair, so you have some people doing the quest in no time at all and others who can literally spend weeks without reward. Hopefully Square fixes it somewhat (there is some evidence that they may have tweaked the drop rate to be a little higher but the system is still poorly designed as it currently stands). These days there are much easier methods of getting a high level weapon so a lot of people are shifting to those instead. It's probably the first instance of true, pure painful grind I've encountered in the game yet. It wouldn't so bad if you could level up another class while you weren't doing it... I have eight out of twelve so far, thinking I might only do relic for the one class...

Love Coerthas. Mor Dhona too. All the high level areas are just beautiful and amazing.

Yeah, post-50 there's still so much more to do! The answer is: everything. You have to do everything.