Your Shadow, in Jungian psychology, is the sum total of everything that's true about you that you don't want to deal with, and so project onto other people: "I'm not the one who is X, it's that other nasty person over there who's X!" Look at fundamentalist preachers ranting about homosexuality as a great example -- you know as well as I do that they're frantically trying not to admit to themselves that they're attracted to men.

Shadow work is the process by which you stop projecting your shadow onto other people and admit the truth about yourself to yourself. There are many ways to do it; I recommend asking people who are into Jungian work (for example, the commentariat on r/Jung over on Reddit) for suggestions.