The shadow is the part of your personality you have repressed throughout the course of your development, and to encounter it is to discover a totally different side of your personality, and thus energies, impulses, and traits, that can help you overcome the troubles and limitations that you currently face in your personal life.
"The shadow, when it is realized, is the source of renewal; the new and productive impulse cannot come from established values of the ego. When there is an impasse, and sterile time in our lives—despite an adequate ego development—we must look to the dark, hitherto unacceptable side which has been at our conscious disposal." (Edward Whitmont)