Active Imagination is a technique developed by C.G. Jung to tap into and become aware of those parts of ourselves that we are not aware of (the unconscious parts of ourselves.) 

The process of active imagination involves several parts.  In the beginning, we become aware of what is hidden (unconscious).  Our conscious awareness is limited compared to what is unconscious.  Dream images, waking images and fantasies, body postures, gestures, somatic/body sensations, feelings and actions, are constantly occurring beneath our conscious awareness.  By passively witnessing our processes, we provide "space" for unconscious contents to be seen and experienced.

Another part of the process, which can occur simultaneously with becoming aware of unconscious material, is giving outer form to what is unconscious.  This can be done in many ways.  The unconscious can be expressed through movement, art, poetry, sandtray, music, movies, collage, photography, verbally, and through many other expressive means.

After giving form to the unconscious, which is a receptive, passive, process, the ego is then employed.  This is the "active" part of active imagination in which we move from a passive perspective to engaging and understanding what has emerged thus enlarging what we know of ourselves. 

Recommended reading:  Chodorow, J. (Ed.). (1997). Jung on Active Imagination.  Princeton, NJ:  Princeton University Press.