This is the 1974th most frequent Russian word.
средний
"Средний" means "middle," "average," or "medium" in English.
Here, 'средний' is an adjective meaning 'medium' or 'average,' describing the difficulty level as not high or low, but in between.
Я выбрал средний уровень сложности задачи.
I chose the medium level of difficulty for the task.
In this example, 'средний' is used as an adjective meaning 'average,' applied to the age to indicate a statistical mean value.
В нашем классе средний возраст учеников — шестнадцать лет.
In our class, the average age of students is sixteen years.
Here, 'средний' means 'middle,' referring to the position in an arrangement of rows.