From Wikipedia:

A Green Man as a name for a sculpture, drawing or other representation of a face surrounded by or made from leaves was coined by Lady Raglan in 1939 [1]. Branches or vines may sprout from the nose, mouth, nostrils or other parts of the face, and these shoots may bear flowers or fruit. Commonly used as a decorative architectural ornament, Green Men are frequently found on carvings in churches and other buildings (both secular and ecclesiastical). "The Green Man" is also a popular name for British public houses and various interpretations of the name appear on inn signs, which sometimes show a full figure rather than just the head.

The Green Man motif has many different faces and variations. Found in many cultures around the world, the Green Man is often related to natural vegetative deities springing up in different cultures throughout the ages. Primarily it is interpreted as a symbol of rebirth, or "renaissance", representing the cycle of growth being reborn anew each spring. Some speculate that the mythology of the Green Man developed independently in the traditions of separate ancient cultures and evolved into the wide variety of examples found throughout history.

From Alancito:

As alluded to above, the Green Man represents rebirth - not in a vague sense, but rather the actual potential in each of us to be spiritually "reborn".

Just as a green plant is fed by light via photosynthesis, so too can humans be spiritually fed by the supreme energy in light. The principle by which this "feeding" occurs is resonance and it is initiated by a change in one's state of attention.

This is explained and illustrated in detail via another "Green Man" ...If, as recommended there, you pay attention to your peripheral vision you will notice that you are actually working with the touch of light upon your eyes - just as a plant works with the touch of light upon its leaves.

The Green Man's presence in churches and cathedrals is hardly surprising when their principle scriptural source of guidance speaks of the power of the Holy Spirit and "The light of the body is the eye: if therefore thine eye be single, thy whole body shall be full of light." ...Enlightenment.