Can I use this line in a song, addressing the fires and the winds:
You fires and winds, please, let the forests be
for them not to get angry anymore and let the forests exist and not destroy them anymore?
Thanks.
The expression let someone/something be generally means stop interfering, though most people would accept its usage in this context.
You have it right. It means to leave the thing alone, as in the "to exist" sense of the verb "to be". Example: "Let it be." (song by the Beatles)