According to Ryan Stone of Ancient Origins,
"The Green Man is believed to have been intended as a symbol of growth and rebirth, the eternal seasonal cycle of the coming of spring and the life of Man. This association stems from the pre-Christian notion that Man was born from nature, as evidenced by various mythological accounts of the way in which the world began, and the idea that Man is directly tied to the fate of nature."
The Green Man is frequently depicted as a man's face, usually ranging from middle aged to elderly, appearing out of the wild of forest trappings. His face is always encompassed by leaves, vines, and flowers, seeming to be literally born from the natural world. However, the slight variations on his images come from the exact way in which the natural world explodes around him. It is common for the Green Man to merely be surrounded by the greenery, hence the name ‘The Green Man’, but there have been archaeological finds of images in which the leaves and vines emanate from his mouth, ears, and other facial orifices, as well as depictions of his face made up completely of nature—facial lines carefully crafted as vines with his skin the very leaves themselves. Man was predominately reliant on nature until recent centuries, so the Green Man as an expression of this close of a relationship also seems likely and a fairly powerful message.
The most common and perhaps obvious interpretation of the Green Man is that of a pagan nature spirit, a symbol of man’s reliance on and union with nature, a symbol of the underlying life-force, and of the renewed cycle of growth each spring. In this respect, it seems likely that he has evolved from older nature deities such as the Celtic Cernunnos and the Greek Pan and Dionysus.
Some have gone so far as to make the argument that the Green Man represents a male counterpart - or son or lover or guardian - to Gaia (or the Earth Mother, or Great Goddess), a figure which has appeared throughout history in almost all cultures. In the 16th Century Cathedral at St-Bertrand de Comminges in southern France, there is even an example of a representation of a winged Earth Mother apparently giving birth to a smiling Green Man.
Survivor from other mythologies: Several other ancient cultures also had green deities, often with some features in common with the Green Man. These include: Humbaba, the ancient Sumerian guardian of the cedar forest, as well as Enkidu, the wild man of the forest in Sumerian mythology, both of which date back to at least 3000 BCE; the Egyptian corn-god Osiris, who is often depicted with a green face representing vegetation and rebirth; Attis, a Phrygian god of vegetation and Nature; the Tibetan Buddhist deity Amoghasiddhi; the Hindu demon Kirtimukha; Tlaloc, the Aztec god of rain, fertility and water; and several others. Some of the features incorporated into ancient representations of these gods reappear centuries later in the Green Man.
Mike Harding tentatively suggests that the symbol may have originated in Asia Minor and been brought to Europe by travelling stone carvers. Harding gives examples of similar figures in Borneo, Nepal, and India (an 8th-century Jain temple in Rajasthan). There are early Romanesque foliate heads in 11th century Templar churches in Jerusalem. Harding also notes that heads from Lebanon and Iraq can be dated to the 2nd century. Tom Cheetham, an authority on Islamic mysticism, identifies Khidr of esoteric Sufism with the Green Man. He develops the idea of the Green Man/Khidr as the principle mediating between the imaginary realm and the physical world.
Despite the range in locations of artifacts of the Green Man, he is most often associated with the society of the Celts, particularly in today's Britain and France, because of the high number of images found in these regions and the stylized way in which he has been portrayed.