Short answers, pulled from the story.
The shadow is an unconscious aspect of personality that does not correspond with a person's ego ideal, causing the ego to resist and project it outward. Carl Jung characterized it as the blind spot of the psyche, and it is also known as the ego-dystonic complex, the repressed id, the shadow aspect, or the shadow archetype.
Carl Jung developed the concept of the shadow within his system of analytical psychology. Jung set the shadow in contrast to the persona and regarded it as unconscious material, made up of the id and a person's own biography, held down beneath the superego's ego-ideal.
The collective shadow is an ancestral version of the shadow carried by the collective experience of the whole human race, rooted in the collective unconscious. It often projects onto figures such as the Devil, added as a fourth member of the Christian trinity described by Paul, or the ancient Egyptian figure Set, and it underlies the split between in-group and out-group behind dehumanization and hate crime.
The shadow typically appears in dreams and visions as a person of the same sex as the dreamer, according to Jungian theory. Its exact form depends heavily on the individual's own lived experience rather than solely on inherited collective material, which is what makes it central to Jungian dream interpretation.
Jung considered merging with the shadow, or being possessed by it, a bad outcome in which the suppressed identity overwrites or controls the ego. He wrote that a man possessed by his shadow is always standing in his own light and falling into his own traps, living below his own level, and used the story of Dr. Jekyll and Mr. Hyde to argue that the conscious personality must integrate the shadow rather than become its slave.
Assimilation is the process of acknowledging the shadow and incorporating parts of it into the ego, producing a stronger and wider consciousness than before. Jungian theory treats the integration of the shadow as the first stage of the analytic process, holding that without it a recognition of the anima and animus is impossible.