This is the 8137th most frequent Russian word.
лесная
"Лесная" means "forest" or "woodland" (adjective).
Here, 'лесная' is an adjective describing the noun 'тропинка' (trail), indicating that the trail is located in or associated with the forest.
Лесная тропинка ведёт к озеру.
The forest trail leads to the lake.
In this sentence, 'лесная' is used as an adjective describing 'флору' (flora), specifying that the flora being studied is related to the forest.
Она изучает лесную флору в ходе экспедиции.
She studies the forest flora during the expedition.
Here, 'лесной' modifies 'деревни' (village), indicating that the village is situated in or near a forest.