The forest is a powerful symbol in folklore and religion, often representing the unknown, danger, and transformation. Entering the wood often marks the beginning of a journey or trial, like in fairy tales (e.g., Little Red Riding Hood, Hansel and Gretel).
It embodies the unpredictable forces of nature and the darker aspects of the psyche—what Jung might call the "shadow self."
In Christian tradition, it's sometimes a symbol of spiritual trial or wandering, like the "dark wood" in Dante’s Inferno, where the pilgrim loses his way.  The forest is also a place of renewal, self-discovery, and magic.  In Celtic and Norse mythology, forests are gateways to the otherworld—inhabited by spirits, faeries, and gods.  The trees themselves are often sacred, living symbols of connection between earth and sky (e.g., Yggdrasil in Norse myth).

Bettelheim, Bruno. The Uses of Enchantment: The Meaning and Importance of Fairy Tales. Vintage Books, 2010.

Eliade, Mircea. The Sacred and the Profane: The Nature of Religion. Translated by Willard R. Trask, Harcourt, 1959.

Zipes, Jack. The Irresistible Fairy Tale: The Cultural and Social History of a Genre. Princeton UP, 2012.

Witness by W. S. Merwin

I want to tell what the forests
were like

I will have to speak
in a forgotten language