The speaker looks at trees and is struck by how beautiful they are, with bright green leaves and branches that reach out like fingers. But the image has a sharper edge too: those fingers "touch and scratch and claw," so the trees seem a little wild, even hungry, as well as lovely.
That mix of wonder and wildness leads to the poem's real wish. The speaker wants to be a tree, because a tree seems "so free." What the speaker imagines is a life with no limits, where one can do "what I please." The idea of freedom builds as the lines grow shorter and sit further apart on the page. They slow down, almost like breathing, until the word "Free" is repeated alone three times at the end.
The mood is admiring and longing, with a hint of restlessness. Judging by the ending, the speaker feels held back in some way, though the poem never says why. The repeated word sounds like both a wish and a quiet ache.