The following page briefly describes what happens when you reach level 50, what does it unlock and how to progress further through the game.
Completing main story quests is enough to reach level 50. Once you reach this level you'll still be able to develop your character and level up, but it will be painfully slow. Once you reach level 50 you should focus on developing your inventory level- ilvl. This is done via Gear Honing, a process which sacrifices resources in exchange for leveling up your current gear (except jewelry). Your next goal will be to reach level 250, then 460 and 803 until you pass level 1,000. This level unlocks more in-game content and more levels in extra activities.
Gaining ilvl levels is important even if your sole focus is completing the main story - after finishing the Shushire continent you'll need ilvl 460 to progress. Then you need ilvl 600 to reach Yorn.
After reaching level 50 and leveling up your equipment you'll unlock new islands with side quests (often providing you with materials needed to develop further) as well as new city activities, such as the Abyssal Dungeon or the Darkness Legion.