This is the 753rd most frequent Russian word.
среднего
"Среднего" means "middle," "average," or "medium" in English, depending on context.
In this sentence, 'среднего' is used as an adjective in Genitive case referring to 'medium' as a size.
Этот стол среднего размера.
This table is of medium size.
Here, 'среднего' modifies 'уровня', indicating an average or middle level, in Genitive case as part of the grammatical construction.
Учитель отмечает ученика среднего уровня.
The teacher is noting the student of medium level.
The term 'среднего' describes 'ума,' indicating average intelligence, utilized in Genitive case.