The idea was described by the ancient Greek philosopher Socrates in about 400 BC, and written by Plato as the Allegory of the Cave. People in the cave see shadows, and do not appreciate the 3-D nature of the objects causing the shadows. They are seeing a 2-D projection of 3-D objects.
A photograph is also a 2-D projection of a 3-D scene. A measurement with a meter stick is a 1-D projection. Other observations can also be viewed as projections of some more complex reality.
Quantum mechanics is the first theory to truly take the cave allegory seriously. It has a theory for how observations correspond to projections, without ever trying to explain what is outside the cave. The theory concocts representations of reality, but is never sure about what the reality is.
Much of the confusion about quantum mechanics occurs when people try to ask about what the theory says about reality. It does not directly say anything about reality. It describes projections of that reality into subspaces, and predicts observations.