This is the 3593rd most frequent German word.
mittleren
"Mittleren" means "middle" or "medium" in English, often used as an attributive adjective.
The word 'mittleren' here is used as an adjective describing the town, specifying its location as 'middle' or 'central' in comparison to others.
Die Familie lebt in einem Haus in der mittleren Stadt.
The family lives in a house in the middle town.
Here, 'mittleren' acts as an adjective describing 'Alter' (age), indicating a specific period in a person's life, considered 'middle' age.
In dem mittleren Alter interessiert man sich oft für andere Hobbys.
In middle age, people often become interested in different hobbies.
In this sentence, 'mittleren' modifies 'Abschnitt' (section), to indicate the part of the book that lies central or between the extremes.