This is the 11320th most frequent Russian word.
дерется
"Дерется" means "fights" (physically).
Here, 'дерётся' means that he is engaging in a physical fight, expressing an action that involves conflict.
Мальчик дерётся на школьном дворе.
The boy is fighting in the school yard.
In this sentence, 'дерётся' conveys an act of fighting with a purpose of defense, showing intention and reason behind the physical fight.
Он дерётся, защищая своих друзей.
He is fighting, defending his friends.
Here, 'дерётся' is used to criticize the tendency to resort to physical means to address issues rather than opting for verbal discussion or other methods.