This is the 14463rd most frequent Spanish word.
árboles
Trees.
In this sentence, 'árboles' is the subject of the sentence and refers to the trees in the park.
Los árboles en el parque son muy antiguos.
The trees in the park are very old.
Here, 'árboles' is the object of the prepositional phrase 'en las copas de', indicating the tops of the trees as a location.
Observé cómo los pájaros anidan en las copas de los árboles.
I observed how the birds nest in the tops of the trees.
In this sentence, 'árboles' is the direct object of the verb 'plantaremos', referring to the trees that are to be planted.