Black Clover's magical filled world is full of stronger mages, but it's always been kind of a general definition of strength. Those with a lot of magic were strong, those without like Asta were deemed weak. There was a wrench thrown into things during the Reincarnation arc in which the elves were more powerful because they had a greater relationship with mana. But there was no real way in which series creator Yuki Tabata had to define how strong characters were to one another. But that's changed with the debut of the Heart Kingdom.
The latest episode of the series brings Asta and the others to the Heart Kingdom as a way to further inspect the threat of the Devils, and they are immediately put up against some strong opponents. But these opponents begin to refer to Asta, Noelle and the others by "stages" and reveal that it's how the Heart Kingdom organizes its strength rankings.
Episode 128 introduces us to "Stages," a system in which the Heart Kingdom organizes those by magical talent and skill. The lowest is a Stage 9 (where Asta finds himself lower than without any magic of his own) with the strongest being Stage 0 like Gaja, the opponent Noella, Finral, and Mimosa found themselves suddenly fighting when Asta is taken directly to the Heart Kingdom's Queen.
This system isn't used in the Clover Kingdom (explaining why we haven't heard about the term yet), but Asta already proves there are flaws in this system. It divides people by rank and skill, but doesn't account for potential. With the help of Secre, Asta is able to escape of the Heart Kingdom Queen's magic and already throws this new power level system for a loop.
This new Stage system will be playing a huge role in the next arc of the series as future foes will be throwing the term around to explain how strong they are and how relatively weak others are in comparison. But with the anime taking some time to expand on an upcoming training period the manga skipped over, maybe there will be a more in depth explanation of this new ranking system in a future episode?
How do you feel about Black Clover's new power level system? Did Black Clover need to explain its power levels? What stages do you think past opponents like Licht and Devil were? Let us know your thoughts in the comments or talk to me directly about all things anime and other cool things @Valdezology on Twitter!