In simple terms, level 50 is when you stop gaining new regular powers or enhancement slots. After training to 50, you start earning XP toward Incarnate content, (basically new, specialized powers which are separate from your regular ones). As you continue to play your level 50 character, your XP bar will still fill, and when you "level up" again, you basically gain some goodies, which can help with the aforementioned Incarnate stuff, or provide other goodies like free costume change or respec tokens...