There is a woman called Mary who had been locked in the black and white room since birth, and she has never seen a colour. And yet she is the world's leading expert on colour. She's read everything there is to read about the neuroscience and the physics of colour. She's read every physical fact about what happened when a human sees colour. And so, one day, when she escaped from her prison, Mary actually sees colour for the first time. She sees a red rose.
The question that Frank Jackson asks in this 1982 thought experiment is: does Mary learn anything new in experiencing that colour? Intuitively, most people want to say, yes, of course she did. She knew everything about red: the physics, the biology, and the brain science. But until the door opened, she didn't know what red was like.
Jackson's point is that there are two different kinds of knowing. There is the knowing you get from reading a book, and there is the knowing you get only from experiencing the actual world. Of course, we are all Mary, trapped in our own black and white rooms. You can read about grief but never experienced loss. You can read about love and never actually be in it. You can read about a culture, a person, a job, a faith, but mistake the reading for the thing.
Jackson's Mary's Room thought experiment argues that you can never fully know a colour until you've experienced it. But more than this, you can never fully know life until you've lived it.