This is the 12910th most frequent Russian word.
лесами
"Лесами" means "by forests" or "through forests" in English.
In this sentence, 'лесами' is used in the instrumental plural form referring to 'forests' as the geographical area being walked through.
Мы гуляли по лесам ранним утром.
We walked through the forests early in the morning.
Here, 'лесами' is used to indicate 'forests' as the locations associated with the action of inhabitation.
Лесами нашей страны обитают редкие животные.
Rare animals inhabit the forests of our country.
In this sentence, 'лесами' describes 'forests' in the sense of the regions affected by the movement of shadows.