This is the 18087th most frequent Russian word.
хвойными
"Хвойными" means "coniferous" in English.
In this sentence, 'хвойными' is an adjective in instrumental plural form, describing the type of trees in the forest.
Лес был покрыт хвойными деревьями.
The forest was covered with coniferous trees.
Here, 'хвойными' again functions as an instrumental plural adjective, emphasizing the type of plants (coniferous) in the park.
Мы прогулялись по парку, засаженному хвойными растениями.
We walked through the park planted with coniferous plants.
In this instance, 'хвойными' describes 'лесами' (forests) in the instrumental plural form, conveying a reason or cause for the fresh air.