This is the 15597th most frequent Russian word.
пригородных
"Suburban" or "pertaining to the suburbs."
The word 'пригородных' is an adjective here describing 'районах' (areas), referring to being related to the suburbs.
Я люблю прогулки в пригородных районах.
I love walks in suburban areas.
Here, 'пригородных' is an adjective modifying 'поездов' (trains), indicating the trains are connected to suburban transportation.
Пассажиры пригородных поездов часто путешествуют на работу.
Suburban train passengers often commute to work.
In this sentence, 'пригородных' functions as an adjective describing 'поселках' (settlements), referring to their location in suburban areas.