This is the 749th most frequent Polish word.
średniej
"Średniej" means "of the average" or "of medium" in English, depending on context.
Here, 'średniej' serves as an adjective in the genitive case, describing the size of the cars.
Średniej wielkości samochody są popularne w miastach.
Medium-sized cars are popular in cities.
In this sentence, 'średniej' modifies 'klasy' and is in the genitive case, indicating the quality of his skills.
On ma średniej klasy umiejętności w piłce nożnej.
He has average-level skills in soccer.
The term 'średniej' is used as a noun in the genitive case, meaning the average value.