A picture plane refers to the physical surface of the painting.
In traditional illusionistic painting using perspective, the picture plane can be thought of as the glass of the notional window through which the viewer looks into the representation of reality that lies beyond. In practice the picture plane is the same as the actual physical surface of the painting.
In modern art the picture plane became a major issue. Formalist theory asserts that a painting is a flat object and that in the interests of truth it should not pretend to be other than flat. In other words, there should be no illusion of three dimensions and so all the elements of the painting should be located on the picture plane.