Does the shadow in shadow work theorised by Carl Jung refer to things we absolutely do not know about ourselves or just the parts of our personality we keep hidden?
If it is hidden in our subconscious, how can we bring it out?
Wikipedia gives a good outline on the Jungian Shadow and its difference to the Freudian version of the shadow.
In Jungian psychology, the "shadow", "Id", or "shadow aspect/archetype" may refer to either an unconscious or subconscious aspect of the personality which the conscious ego does not identify in itself, or even the entirety of the unconscious/subconscious, i.e., everything of which a person is not fully conscious. In short, the shadow is the "dark side" as opposed to being in the light of the conscious.
Because one tends to reject or remain ignorant of the least desirable aspects of one's personality, the shadow is largely negative. There are, however, positive aspects that may also remain hidden in one's shadow (especially in people with low self-esteem, anxieties, and false beliefs).
Because the shadow is in the subconscious/unconscious you can be ignorant of it as you are not conscious of it. Sometimes the "shadow" can make itself known (see the second part of my answer) and therefore at that time you are conscious of that part of the shadow. You can choose to ignore it and repress it back to your subconscious or you can acknowledge it and deal with it, assimilating it into your conscious.
You might have a hidden talent which is repressed by anxieties or self-esteem issues for example, or you may have biases within your thoughts or actions which you are unconsciously aware of.
The shadow personifies everything that the subject refuses to acknowledge about himself (Jung, 1968 p.284).
We cannot always "bring it out". It will manifest itself by itself. The shadow can appear in dreams or when the person projects it onto another person (Papadopoulos, 2006).
You can try to become aware of your "shadow" by being consciously aware of your thoughts, feelings and actions; and where necessary, analysing them either by finding out if your thoughts and feelings are founded on your own or by speaking about your concerns with a trusted person or therapist.
The books within the references may help more with this question.
Jung, C. G. (1968) The Archetypes and the Collective Unconscious Princeton, NJ: Princeton University Press
Free PDF copy available at https://archive.org/download/collectedworksof91cgju/collectedworksof91cgju.pdf
Papadopoulos, R. K. (Ed.). (2006). The handbook of Jungian psychology: Theory, practice and applications. New York, NY: Routledge.
for the definition of "the shadow" we firstly go throw the basic difference between Jung and Freud which is the collective unconscious or the storehouse of memories inherited from the common ancestors of the whole human race which contains the archetypes that we could define it as emotionally charged images and thought forms that have universal meaning. one of those archetypes we have is the "shadow" exactly defined as a prehistoric fear of wild animals, which represents the animal side of the human nature.